removed unnecessary mapping for on options
removed unnecessary mapping for on options

Right before that in `assert_valid_transaction_action` method we make
sure that `options[:on]` contains values from `ACTIONS` array
(`[:create, :destroy, :update]`) and nothing more (i.e. it could not
contain strings or something else, otherwise the error is raised).

Also I've polished some docs.

ngpestelos/rails,xlymian/rails,mechanicles/rails,mijoharas/rails,betesh/rails,Sen-Zhang/rails,marklocklear/rails,arjes/rails,kddeisz/rails,richseviora/rails,baerjam/rails,illacceptanything/illacceptanything,yalab/rails,rafaelfranca/omg-rails,gcourtemanche/rails,vipulnsward/rails,lucasmazza/rails,fabianoleittes/rails,Envek/rails,rossta/rails,schuetzm/rails,untidy-hair/rails,Envek/rails,mohitnatoo/rails,gcourtemanche/rails,jeremy/rails,baerjam/rails,BlakeWilliams/rails,kddeisz/rails,ledestin/rails,MichaelSp/rails,vassilevsky/rails,BlakeWilliams/rails,printercu/rails,arunagw/rails,88rabbit/newRails,richseviora/rails,kenta-s/rails,tjschuck/rails,notapatch/rails,ttanimichi/rails,schuetzm/rails,georgeclaghorn/rails,yawboakye/rails,odedniv/rails,esparta/rails,kirs/rails-1,Stellenticket/rails,palkan/rails,betesh/rails,rafaelfranca/omg-rails,Erol/rails,esparta/rails,samphilipd/rails,jeremy/rails,coreyward/rails,eileencodes/rails,ngpestelos/rails,illacceptanything/illacceptanything,prathamesh-sonpatki/rails,kamipo/rails,flanger001/rails,illacceptanything/illacceptanything,Vasfed/rails,tjschuck/rails,88rabbit/newRails,flanger001/rails,gavingmiller/rails,brchristian/rails,riseshia/railsguides.kr,yahonda/rails,shioyama/rails,lucasmazza/rails,Edouard-chin/rails,printercu/rails,notapatch/rails,arunagw/rails,kaspth/rails,gauravtiwari/rails,kmcphillips/rails,repinel/rails,travisofthenorth/rails,pvalena/rails,elfassy/rails,lcreid/rails,Vasfed/rails,tijwelch/rails,bolek/rails,mtsmfm/railstest,repinel/rails,vassilevsky/rails,illacceptanything/illacceptanything,illacceptanything/illacceptanything,illacceptanything/illacceptanything,kamipo/rails,maicher/rails,stefanmb/rails,bogdanvlviv/rails,bogdanvlviv/rails,mijoharas/rails,mathieujobin/reduced-rails-for-travis,riseshia/railsguides.kr,illacceptanything/illacceptanything,untidy-hair/rails,travisofthenorth/rails,sergey-alekseev/rails,lcreid/rails,Stellenticket/rails,sealocal/rails,prathamesh-sonpatki/rails,vipulnsward/rails,bradleypriest/rails,printercu/rails,printercu/rails,EmmaB/rails-1,ttanimichi/rails,Spin42/rails,MSP-Greg/rails,88rabbit/newRails,mohitnatoo/rails,sealocal/rails,gfvcastro/rails,ttanimichi/rails,joonyou/rails,elfassy/rails,shioyama/rails,alecspopa/rails,fabianoleittes/rails,MichaelSp/rails,gfvcastro/rails,felipecvo/rails,rossta/rails,bolek/rails,yalab/rails,kddeisz/rails,hanystudy/rails,Vasfed/rails,lucasmazza/rails,notapatch/rails,tjschuck/rails,ledestin/rails,rails/rails,tgxworld/rails,flanger001/rails,gauravtiwari/rails,rafaelfranca/omg-rails,MSP-Greg/rails,maicher/rails,yalab/rails,georgeclaghorn/rails,assain/rails,mohitnatoo/rails,palkan/rails,bogdanvlviv/rails,eileencodes/rails,yasslab/railsguides.jp,starknx/rails,tgxworld/rails,sealocal/rails,betesh/rails,marklocklear/rails,tijwelch/rails,Erol/rails,amoody2108/TechForJustice,kachick/rails,rossta/rails,koic/rails,sergey-alekseev/rails,aditya-kapoor/rails,Envek/rails,kmayer/rails,rbhitchcock/rails,palkan/rails,yhirano55/rails,alecspopa/rails,yawboakye/rails,mijoharas/rails,illacceptanything/illacceptanything,repinel/rails,travisofthenorth/rails,BlakeWilliams/rails,pschambacher/rails,flanger001/rails,felipecvo/rails,kamipo/rails,bradleypriest/rails,gfvcastro/rails,Spin42/rails,lcreid/rails,koic/rails,MichaelSp/rails,aditya-kapoor/rails,alecspopa/rails,Sen-Zhang/rails,Edouard-chin/rails,travisofthenorth/rails,yahonda/rails,schuetzm/rails,arjes/rails,Stellenticket/rails,kmcphillips/rails,kachick/rails,esparta/rails,palkan/rails,mohitnatoo/rails,rails/rails,jeremy/rails,eileencodes/rails,iainbeeston/rails,mtsmfm/rails,lcreid/rails,schuetzm/rails,voray/rails,matrinox/rails,yhirano55/rails,prathamesh-sonpatki/rails,yasslab/railsguides.jp,vassilevsky/rails,yalab/rails,xlymian/rails,jeremy/rails,deraru/rails,koic/rails,Spin42/rails,aditya-kapoor/rails,yhirano55/rails,arunagw/rails,rails/rails,deraru/rails,assain/rails,iainbeeston/rails,eileencodes/rails,mtsmfm/rails,ngpestelos/rails,ledestin/rails,amoody2108/TechForJustice,samphilipd/rails,prathamesh-sonpatki/rails,mechanicles/rails,marklocklear/rails,iainbeeston/rails,BlakeWilliams/rails,esparta/rails,bradleypriest/rails,mechanicles/rails,vipulnsward/rails,arunagw/rails,aditya-kapoor/rails,kmcphillips/rails,yawboakye/rails,tjschuck/rails,kaspth/rails,Envek/rails,richseviora/rails,Edouard-chin/rails,notapatch/rails,shioyama/rails,gcourtemanche/rails,sergey-alekseev/rails,iainbeeston/rails,pvalena/rails,kaspth/rails,tgxworld/rails,stefanmb/rails,gfvcastro/rails,kachick/rails,gavingmiller/rails,utilum/rails,hanystudy/rails,rbhitchcock/rails,joonyou/rails,fabianoleittes/rails,pvalena/rails,rbhitchcock/rails,samphilipd/rails,starknx/rails,kirs/rails-1,maicher/rails,georgeclaghorn/rails,riseshia/railsguides.kr,baerjam/rails,coreyward/rails,xlymian/rails,brchristian/rails,gauravtiwari/rails,joonyou/rails,kmayer/rails,matrinox/rails,illacceptanything/illacceptanything,MSP-Greg/rails,joonyou/rails,stefanmb/rails,assain/rails,illacceptanything/illacceptanything,Stellenticket/rails,yhirano55/rails,Vasfed/rails,Erol/rails,vipulnsward/rails,mathieujobin/reduced-rails-for-travis,yasslab/railsguides.jp,arjes/rails,kmcphillips/rails,georgeclaghorn/rails,gavingmiller/rails,Edouard-chin/rails,illacceptanything/illacceptanything,Erol/rails,fabianoleittes/rails,utilum/rails,pvalena/rails,illacceptanything/illacceptanything,coreyward/rails,yasslab/railsguides.jp,voray/rails,matrinox/rails,illacceptanything/illacceptanything,EmmaB/rails-1,mtsmfm/railstest,kddeisz/rails,kenta-s/rails,baerjam/rails,illacceptanything/illacceptanything,kirs/rails-1,kmayer/rails,voray/rails,EmmaB/rails-1,hanystudy/rails,riseshia/railsguides.kr,bolek/rails,tgxworld/rails,untidy-hair/rails,untidy-hair/rails,utilum/rails,bogdanvlviv/rails,yawboakye/rails,deraru/rails,odedniv/rails,odedniv/rails,MSP-Greg/rails,pschambacher/rails,amoody2108/TechForJustice,repinel/rails,brchristian/rails,mtsmfm/railstest,elfassy/rails,illacceptanything/illacceptanything,rails/rails,kenta-s/rails,shioyama/rails,tijwelch/rails,assain/rails,deraru/rails,betesh/rails,felipecvo/rails,mathieujobin/reduced-rails-for-travis,starknx/rails,utilum/rails,yahonda/rails,pschambacher/rails,Sen-Zhang/rails,mechanicles/rails,mtsmfm/rails,yahonda/rails