Add doc for absURL and relURL
Add doc for absURL and relURL

Also group URL related functions in doc.

jygastaud/hugo,fedelibre/hugo,ifraixedes/hugo,abourget/hugo,abourget/hugo,tubo028/hugo,tischda/hugo,mdhender/hugo,melo/hugo,Zenithar/hugo,ffrizzo/hugo,mdhender/hugo,ryanclarke/hugo,coderzh/hugo,ryanclarke/hugo,fedelibre/hugo,halostatue/hugo,tischda/hugo,badoet/hugo,melo/hugo,tubo028/hugo,digitalcraftsman/hugo,tjyang/hugo,soundrussian/hugo,sprig/hugo,gohugoio/hugo,vbatoufflet/hugo,digitalcraftsman/hugo,anthonyfok/hugo,mayah/hugo,bogem/hugo,vbatoufflet/hugo,ifraixedes/hugo,fhergomez/hugo,kujohn/hugo,tatsushid/hugo,jygastaud/hugo,bramp/hugo,ffrizzo/hugo,anthonyfok/hugo,tjyang/hugo,fedelibre/hugo,MarkDBlackwell/hugo,mdhender/hugo,fhergomez/hugo,anthonyfok/hugo,kujohn/hugo,gohugoio/hugo,mitchchn/hugo,MarkDBlackwell/hugo,soundrussian/hugo,bramp/hugo,bogem/hugo,mitchchn/hugo,gohugoio/hugo,coderzh/hugo,mdhender/hugo,sprig/hugo,tatsushid/hugo,fedelibre/hugo,badoet/hugo,halostatue/hugo,derekperkins/hugo,mayah/hugo,derekperkins/hugo,Zenithar/hugo