Add guides for `required` model generator option.

Related to this PR #16062

[ci skip]
Merge pull request #17988 from caike/master

Add guides for `required` model generator option
Adding information about 'allow_nil' option for 'numericality' [ci skip]
Merge pull request #17987 from andreynering/ar-validation-doc

Adding information about 'allow_nil' option for 'numericality' [ci skip]
[ci skip] fix typo in MessageVerifier#verify docs
Merge pull request #17989 from y-yagi/patch-1

[ci skip] fix typo in MessageVerifier#verify docs