Update configuring guide with I18n options regarding available locales
Update configuring guide with I18n options regarding available locales

Add information about 'available_locales' and 'enforce_available_locales'
to the configuring guide.

Closes #13202. [ci skip]

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