Update RecordNotFound exception cases to include a message with the
Model that the Record was not found in.
A warning line should look like a warning section

[ci skip]
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
Delegation works with reserved words passed to `:to`

Fixes #16956.
 Updated comment about features of fixtures

 - Added a note about some features which are not dependent on
   auto-generated ID's.
 - [ci skip]
Merge pull request #16964 from Agis-/issue-16956

Make delegation work with the reserved words passed to `:to`
Merge pull request #16975 from lethalbrains/master

[ci skip] ActionRecord CHANGELOG docs fixes
Merge pull request #16973 from prathamesh-sonpatki/fixtures-features

 Updated comment about features of fixtures
Standardize on `Rails.application` [ci skip]

This seems to be the style settled on in most of the templates.
[ci skip] Reword a comment in concern's documentation

Nothing biggie. Skimmed through `ActiveSupport::Concern` docs these days
and this one comment seemed a bit off.