Add guides for `required` model generator option.
Add guides for `required` model generator option.

Related to this PR #16062

[ci skip]

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