Fix active_record_validations.md document, `:save` for `on:` validation helper was never available
Fix active_record_validations.md document, `:save` for `on:` validation helper was never available

According to the guide, ":save" value for the "on:" validation helper
was available like below

    validates :name, presence: true, on: :save

but this was never available according to the implementation of the
valid? method, which is below

    # Runs all the validations within the specified context. Returns
+true+ if
    # no errors are found, +false+ otherwise.
    #
    # If the argument is +false+ (default is +nil+), the context is set
to <tt>:create</tt> if
    # <tt>new_record?</tt> is +true+, and to <tt>:update</tt> if it is
not.
    #
    # Validations with no <tt>:on</tt> option will run no matter the
context. Validations with
    # some <tt>:on</tt> option will only run in the specified context.
    def valid?(context = nil)
      context ||= (new_record? ? :create : :update)
      output = super(context)
      errors.empty? && output
    end

So the documentation was always wrong since the PR proposed by
@neerajdotname  ( #10287 ) was rejected.

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