improve the specs to better document the shipping_calculator
improve the specs to better document the shipping_calculator

Related to #4041

tancnle/spree,wolfieorama/spree,camelmasa/spree,orenf/spree,edgward/spree,gregoryrikson/spree-sample,adaddeo/spree,softr8/spree,camelmasa/spree,raow/spree,locomotivapro/spree,siddharth28/spree,carlesjove/spree,progsri/spree,JuandGirald/spree,richardnuno/solidus,trigrass2/spree,vinayvinsol/spree,mleglise/spree,omarsar/spree,vmatekole/spree,archSeer/spree,jsurdilla/solidus,grzlus/solidus,grzlus/spree,ramkumar-kr/spree,grzlus/solidus,DarkoP/spree,pulkit21/spree,berkes/spree,SadTreeFriends/spree,azclick/spree,locomotivapro/spree,tancnle/spree,biagidp/spree,kewaunited/spree,dafontaine/spree,njerrywerry/spree,woboinc/spree,firman/spree,assembledbrands/spree,jparr/spree,net2b/spree,assembledbrands/spree,alepore/spree,bjornlinder/Spree,sfcgeorge/spree,jparr/spree,shaywood2/spree,alepore/spree,patdec/spree,jimblesm/spree,firman/spree,Ropeney/spree,gregoryrikson/spree-sample,jordan-brough/solidus,derekluo/spree,athal7/solidus,camelmasa/spree,FadliKun/spree,groundctrl/spree,woboinc/spree,surfdome/spree,trigrass2/spree,imella/spree,jimblesm/spree,agient/agientstorefront,shekibobo/spree,azranel/spree,dafontaine/spree,degica/spree,freerunningtech/spree,mindvolt/spree,keatonrow/spree,brchristian/spree,scottcrawford03/solidus,builtbybuffalo/spree,jsurdilla/solidus,vulk/spree,Nevensoft/spree,biagidp/spree,vmatekole/spree,DynamoMTL/spree,tailic/spree,Hates/spree,sliaquat/spree,assembledbrands/spree,project-eutopia/spree,LBRapid/spree,Migweld/spree,jaspreet21anand/spree,grzlus/spree,omarsar/spree,shekibobo/spree,brchristian/spree,jspizziri/spree,robodisco/spree,pervino/solidus,scottcrawford03/solidus,TimurTarasenko/spree,ayb/spree,ayb/spree,caiqinghua/spree,CJMrozek/spree,hifly/spree,richardnuno/solidus,bricesanchez/spree,groundctrl/spree,AgilTec/spree,zamiang/spree,alvinjean/spree,lsirivong/solidus,APohio/spree,jordan-brough/spree,gautamsawhney/spree,archSeer/spree,moneyspyder/spree,njerrywerry/spree,pervino/spree,dotandbo/spree,fahidnasir/spree,devilcoders/solidus,omarsar/spree,jaspreet21anand/spree,lsirivong/solidus,moneyspyder/spree,tancnle/spree,kewaunited/spree,caiqinghua/spree,carlesjove/spree,Migweld/spree,volpejoaquin/spree,Engeltj/spree,nooysters/spree,tancnle/spree,surfdome/spree,tesserakt/clean_spree,raow/spree,CJMrozek/spree,ckk-scratch/solidus,vcavallo/spree,quentinuys/spree,DarkoP/spree,degica/spree,ahmetabdi/spree,JDutil/spree,SadTreeFriends/spree,TrialGuides/spree,pervino/spree,rbngzlv/spree,CiscoCloud/spree,rakibulislam/spree,AgilTec/spree,watg/spree,Ropeney/spree,welitonfreitas/spree,miyazawatomoka/spree,tomash/spree,derekluo/spree,hifly/spree,adaddeo/spree,yomishra/pce,ayb/spree,devilcoders/solidus,freerunningtech/spree,piousbox/spree,Machpowersystems/spree_mach,alvinjean/spree,net2b/spree,Ropeney/spree,JDutil/spree,miyazawatomoka/spree,azclick/spree,KMikhaylovCTG/spree,LBRapid/spree,delphsoft/spree-store-ballchair,dandanwei/spree,devilcoders/solidus,jeffboulet/spree,vulk/spree,Senjai/spree,lyzxsc/spree,jsurdilla/solidus,hifly/spree,robodisco/spree,xuewenfei/solidus,bonobos/solidus,athal7/solidus,Lostmyname/spree,jaspreet21anand/spree,jimblesm/spree,ckk-scratch/solidus,maybii/spree,abhishekjain16/spree,tomash/spree,madetech/spree,CJMrozek/spree,dotandbo/spree,mindvolt/spree,brchristian/spree,radarseesradar/spree,azclick/spree,FadliKun/spree,locomotivapro/spree,dotandbo/spree,njerrywerry/spree,pulkit21/spree,vmatekole/spree,Arpsara/solidus,surfdome/spree,karlitxo/spree,APohio/spree,shioyama/spree,sideci-sample/sideci-sample-spree,sfcgeorge/spree,knuepwebdev/FloatTubeRodHolders,patdec/spree,freerunningtech/spree,calvinl/spree,carlesjove/spree,piousbox/spree,Arpsara/solidus,pervino/spree,reinaris/spree,ujai/spree,TrialGuides/spree,APohio/spree,Kagetsuki/spree,vinsol/spree,jeffboulet/spree,Kagetsuki/spree,fahidnasir/spree,alepore/spree,vinsol/spree,jparr/spree,xuewenfei/solidus,Senjai/solidus,delphsoft/spree-store-ballchair,Hawaiideveloper/shoppingcart,abhishekjain16/spree,pulkit21/spree,madetech/spree,tomash/spree,PhoenixTeam/spree_phoenix,Senjai/solidus,shaywood2/spree,watg/spree,reinaris/spree,zaeznet/spree,adaddeo/spree,CiscoCloud/spree,reinaris/spree,grzlus/solidus,adaddeo/spree,progsri/spree,patdec/spree,yushine/spree,orenf/spree,FadliKun/spree,gautamsawhney/spree,bonobos/solidus,mleglise/spree,jordan-brough/solidus,vulk/spree,rajeevriitm/spree,berkes/spree,sunny2601/spree,CiscoCloud/spree,Nevensoft/spree,jspizziri/spree,knuepwebdev/FloatTubeRodHolders,quentinuys/spree,shaywood2/spree,progsri/spree,madetech/spree,alejandromangione/spree,welitonfreitas/spree,volpejoaquin/spree,JuandGirald/spree,Hawaiideveloper/shoppingcart,wolfieorama/spree,jasonfb/spree,xuewenfei/solidus,jimblesm/spree,dafontaine/spree,DynamoMTL/spree,thogg4/spree,Migweld/spree,sliaquat/spree,AgilTec/spree,wolfieorama/spree,Hates/spree,piousbox/spree,tesserakt/clean_spree,sunny2601/spree,shaywood2/spree,vinayvinsol/spree,quentinuys/spree,caiqinghua/spree,odk211/spree,net2b/spree,siddharth28/spree,delphsoft/spree-store-ballchair,vinsol/spree,StemboltHQ/spree,sideci-sample/sideci-sample-spree,DynamoMTL/spree,trigrass2/spree,ramkumar-kr/spree,DynamoMTL/spree,keatonrow/spree,alvinjean/spree,Boomkat/spree,builtbybuffalo/spree,JDutil/spree,jordan-brough/solidus,StemboltHQ/spree,radarseesradar/spree,Senjai/spree,zaeznet/spree,lyzxsc/spree,vulk/spree,Engeltj/spree,NerdsvilleCEO/spree,trigrass2/spree,CiscoCloud/spree,Boomkat/spree,ahmetabdi/spree,CJMrozek/spree,Boomkat/spree,jspizziri/spree,gregoryrikson/spree-sample,lsirivong/spree,Nevensoft/spree,sliaquat/spree,odk211/spree,miyazawatomoka/spree,lsirivong/spree,ahmetabdi/spree,zamiang/spree,azranel/spree,hoanghiep90/spree,maybii/spree,jeffboulet/spree,xuewenfei/solidus,LBRapid/spree,TimurTarasenko/spree,derekluo/spree,Senjai/solidus,Hawaiideveloper/shoppingcart,lsirivong/solidus,vcavallo/spree,agient/agientstorefront,orenf/spree,quentinuys/spree,scottcrawford03/solidus,joanblake/spree,reinaris/spree,SadTreeFriends/spree,Lostmyname/spree,joanblake/spree,tomash/spree,Arpsara/solidus,omarsar/spree,pervino/solidus,beni55/spree,KMikhaylovCTG/spree,volpejoaquin/spree,jordan-brough/solidus,sunny2601/spree,alvinjean/spree,ujai/spree,Kagetsuki/spree,tailic/spree,lsirivong/solidus,vinsol/spree,gregoryrikson/spree-sample,hoanghiep90/spree,richardnuno/solidus,Engeltj/spree,mindvolt/spree,mindvolt/spree,watg/spree,Mayvenn/spree,wolfieorama/spree,dotandbo/spree,mleglise/spree,surfdome/spree,cutefrank/spree,miyazawatomoka/spree,KMikhaylovCTG/spree,keatonrow/spree,vinayvinsol/spree,groundctrl/spree,biagidp/spree,yushine/spree,cutefrank/spree,lyzxsc/spree,Lostmyname/spree,joanblake/spree,yiqing95/spree,PhoenixTeam/spree_phoenix,JuandGirald/spree,lsirivong/spree,pervino/solidus,jeffboulet/spree,jspizziri/spree,project-eutopia/spree,softr8/spree,imella/spree,TimurTarasenko/spree,vmatekole/spree,thogg4/spree,vinayvinsol/spree,forkata/solidus,priyank-gupta/spree,Boomkat/spree,grzlus/solidus,Hawaiideveloper/shoppingcart,hoanghiep90/spree,welitonfreitas/spree,siddharth28/spree,softr8/spree,tesserakt/clean_spree,radarseesradar/spree,dandanwei/spree,yomishra/pce,yiqing95/spree,calvinl/spree,zamiang/spree,kewaunited/spree,RatioClothing/spree,progsri/spree,edgward/spree,knuepwebdev/FloatTubeRodHolders,yiqing95/spree,bjornlinder/Spree,jasonfb/spree,bjornlinder/Spree,forkata/solidus,agient/agientstorefront,hoanghiep90/spree,ramkumar-kr/spree,ahmetabdi/spree,forkata/solidus,rakibulislam/spree,gautamsawhney/spree,reidblomquist/spree,richardnuno/solidus,SadTreeFriends/spree,jasonfb/spree,mleglise/spree,priyank-gupta/spree,project-eutopia/spree,rajeevriitm/spree,bonobos/solidus,RatioClothing/spree,firman/spree,orenf/spree,Senjai/solidus,grzlus/spree,madetech/spree,beni55/spree,dafontaine/spree,Hates/spree,volpejoaquin/spree,keatonrow/spree,derekluo/spree,DarkoP/spree,Machpowersystems/spree_mach,Arpsara/solidus,nooysters/spree,thogg4/spree,sideci-sample/sideci-sample-spree,shioyama/spree,zaeznet/spree,carlesjove/spree,APohio/spree,JuandGirald/spree,rajeevriitm/spree,brchristian/spree,AgilTec/spree,joanblake/spree,maybii/spree,shioyama/spree,delphsoft/spree-store-ballchair,bricesanchez/spree,scottcrawford03/solidus,Nevensoft/spree,rajeevriitm/spree,forkata/solidus,radarseesradar/spree,camelmasa/spree,reidblomquist/spree,rakibulislam/spree,hifly/spree,Hates/spree,robodisco/spree,azranel/spree,NerdsvilleCEO/spree,berkes/spree,jasonfb/spree,jsurdilla/solidus,Senjai/spree,rbngzlv/spree,gautamsawhney/spree,sfcgeorge/spree,TrialGuides/spree,abhishekjain16/spree,bonobos/solidus,project-eutopia/spree,Engeltj/spree,cutefrank/spree,kewaunited/spree,agient/agientstorefront,pervino/spree,builtbybuffalo/spree,rbngzlv/spree,beni55/spree,karlitxo/spree,ujai/spree,tesserakt/clean_spree,yushine/spree,dandanwei/spree,useiichi/spree,useiichi/spree,RatioClothing/spree,edgward/spree,PhoenixTeam/spree_phoenix,rbngzlv/spree,groundctrl/spree,TimurTarasenko/spree,imella/spree,njerrywerry/spree,Mayvenn/spree,azranel/spree,Lostmyname/spree,nooysters/spree,locomotivapro/spree,fahidnasir/spree,azclick/spree,KMikhaylovCTG/spree,NerdsvilleCEO/spree,TrialGuides/spree,pervino/solidus,beni55/spree,Migweld/spree,yomishra/pce,softr8/spree,odk211/spree,JDutil/spree,lyzxsc/spree,Machpowersystems/spree_mach,jordan-brough/spree,maybii/spree,calvinl/spree,sliaquat/spree,robodisco/spree,ayb/spree,yushine/spree,dandanwei/spree,lsirivong/spree,shekibobo/spree,woboinc/spree,sfcgeorge/spree,edgward/spree,jparr/spree,degica/spree,builtbybuffalo/spree,StemboltHQ/spree,reidblomquist/spree,bricesanchez/spree,tailic/spree,Kagetsuki/spree,Mayvenn/spree,alejandromangione/spree,welitonfreitas/spree,net2b/spree,useiichi/spree,sunny2601/spree,fahidnasir/spree,nooysters/spree,firman/spree,alejandromangione/spree,piousbox/spree,grzlus/spree,DarkoP/spree,alejandromangione/spree,calvinl/spree,patdec/spree,vcavallo/spree,archSeer/spree,caiqinghua/spree,raow/spree,Mayvenn/spree,NerdsvilleCEO/spree,Ropeney/spree,pulkit21/spree,rakibulislam/spree,jaspreet21anand/spree,PhoenixTeam/spree_phoenix,athal7/solidus,ckk-scratch/solidus,raow/spree,siddharth28/spree,useiichi/spree,archSeer/spree,moneyspyder/spree,reidblomquist/spree,moneyspyder/spree,devilcoders/solidus,zamiang/spree,karlitxo/spree,athal7/solidus,karlitxo/spree,zaeznet/spree,FadliKun/spree,ckk-scratch/solidus,cutefrank/spree,abhishekjain16/spree,berkes/spree,priyank-gupta/spree,yiqing95/spree,priyank-gupta/spree,jordan-brough/spree,thogg4/spree,ramkumar-kr/spree,odk211/spree,vcavallo/spree,shekibobo/spree