A warning line should look like a warning section

[ci skip]
Cleanup loaded features and constants after dependency tests.
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]
[ci skip] add guide for ActiveModel::SecurePassword
'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.
Document that the default for `rails server -b` has changed

Fixes #16578
Merge pull request #16984 from chancancode/fix_16578

Document that the default for `rails server -b` has changed
Fix assertions in AR::TestCase::AttributeMethodsTest

This test has always been green because it uses "assert" and the first
argument is an truthy class/object.
Merge pull request #16998 from yuki24/fix-false-assertion

Fix false assertions in AR::TestCase::AttributeMethodsTest
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