'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
Standardize on `Rails.application` [ci skip]

This seems to be the style settled on in most of the templates.
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.
[ci skip] modify the description of production log level default
Merge pull request #16988 from y-yagi/chg_default_log_level

[ci skip] modify the description of production log level default
Fix typo [ci skip]
Merge pull request #16994 from yuki24/fix-typo-in-ar-attribute-method-test

[ci skip] Fix typo in activerecord/test/cases/attribute_methods_test.rb:737
Merge pull request #16979 from square/standardize-call-style

Standardize on `Rails.application` [ci skip]
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.