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.
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.
[ci skip] Remove reference to config.threadsafe! in guides/configuriing.md
remove extra # in css
Merge pull request #17059 from bronzle/remove_threadsafe_from_guides

[ci skip] Remove reference to config.threadsafe! in guides/configuring.md
Merge pull request #17061 from bronzle/remove_extra_#

remove extra # in css
Revert "Merge pull request #16966 from why-el/symbolize-path-params"

This reverts commit 9d05d6de52871e57bfbf54a60de005e8a5f5b0e4, reversing
changes made to 0863c9248fd47a15e88e05ce4fcd80966684c0e3.

The change in the behaviour reported at #16958 doesn't exist since 4.0
and 4.1 works in the same way
Add +variants to ActionView::FileSystemResolver documentation.
Merge pull request #17070 from felipecvo/master

Add +variants to ActionView::FileSystemResolver documentation.
Don’t require already required modules

abstract_unit.rb requires `action_controller` which [already includes the following lines of code](https://github.com/rails/rails/blob/64fcdce1d3a6a8768ab17f3be144270456814f82/actionpack/lib/action_controller.rb#L2L3):

```ruby
require 'abstract_controller'
require 'action_dispatch'
```
Merge pull request #17071 from claudiob/don-t-require-action-dispatch-twice

Don’t require already required modules
Preparing for 4.2.0.beta2 release
Add documentation about `config.action_dispatch.rescue_responses`

[ci skip]