Add doc for `:touch` option of AR::Base#save

ActiveRecord::Base `save` and `save!` take an option boolean
`:touch` parameter since #18225 (stems from #18202).

This commit document that parameter.

[ci skip]
Merge pull request #18249 from claudiob/add-doc-for-save-with-touch

Add doc for `:touch` option of AR::Base#save
Pass through `assert_select` testing guide [ci skip]
Update link to rails-dom-testing documentation [ci skip]
Remove pre-4.2 comment about missing `assert_select` [ci skip]
Merge branch 'master' of github.com:rails/rails [ci skip]