Add a note on custom validation contexts.

The documentation on `:on` for validations was inconsistent, and most
only referenced the `:create` and `:update` contexts. I fixed those to
be consistent with the documentation on `AM::Validations.validates`,
which seemed to have the best docs.

[ci skip]
'Reversible' ->active_record_migrations [ci skip]

Reversible method documentation moved from migrations.md  to active_record_migrations.md
'Revert' ->active_record_migrations [ci skip]

Revert method documentation moved from migrations.md  to active_record_migrations.md
[ci skip] Fix some typos, normalize sentence.

Follow up of #16722.

https://github.com/rails/rails/pull/16722

also normalize all instance gained methods’ sentence.
Changes default render behavior from file to template.
form_helpers: demonstrating there is also a collection_select method in form builder
[ci skip] Fix code snippet display in Action Mailer Basics guide.
Consistently capitalize ETag.
Consistently markup etag options.
Cleaner queuing of jobs using Delayed Job
Skip isolated tests for PRs

They're valuable, but the types of errors they catch are fairly rare: we
can address them post-merge when they show up.
Update error message for validate method

- Improve the error message by suggesting that the user may have
  intended to call validates instead of validate method.
Fix another false assertions

 * The assertions in AS::Duration don't actually assert.
 * The assertion in Railtie will pass even when `eager_load_namespaces`
   doesn't include `AppTemplate::Application` if `Rails.application` is
   truthy.

For more details, see here:

 * https://github.com/rails/rails/pull/16998
 * https://github.com/rails/rails/pull/17000
Wrap Adequate Record release notes at 80 chars [ci skip]
Wrap it up! :trollface: [ci skip]
Merge pull request #17000 from yuki24/fix-false-assertions

Fix another false assertions
Remove wrapping <div> in form helpers from doc

[ci skip]

89ff1f8 and 1de258e6 removed from the HTML generated by the form helpers
the <div> that was wrapping the field elements inside the <form>.

This commit updates the documentation of the methods to reflect the
two commits above.
Merge pull request #17001 from claudiob/remove-wrapping-div-from-form-helpers-doc

Remove wrapping <div> in form helpers from doc [ci skip]
Allow attaching files while the mail view is rendered
Merge pull request #16974 from silp-com/mail_was_called

ActionMailer: mark mail as called after instead of before processing it