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
Follow up to #16613

Since we want this flag to be enabled anytime we are running the tests
under JRuby, let's enable this at the Rakefile level so people get the
performance boost on their local checkout.

Moreover, we avoid having to update this particular line anytime the
option changes on the JRuby side.

The only drawback is that we have to define it in every Rakefile but
there's no big deal, this is already the case for other options.
Responder was removed at ee77770d57de9da87b05a2fe84b9d46ec6852c62
Remove RUNNING UNIT TESTS from ActionPack

[ci skip]

Following discussion with @senny https://github.com/rails/rails/pull/17100#issuecomment-57285273
it only makes sense to keep this file form projects that require
extra instructions.
Merge pull request #17110 from claudiob/remove-running-tests

Remove RUNNING UNIT TESTS from ActionPack
Added processing of `nil` to `highlight` helper method
Refactor callback setup in to use lambda instead of eval
Update single table inheritance link anchor

The current one is old-of-date and doesn't make the browser navigation to the correct heading.
Merge pull request #17118 from zhangsu/patch-1

Update single table inheritance link anchor
Generate the application pointing to rails-dom-testing master

It is temporary until we do the final release
Merge pull request #17114 from pabloh/replace_use_of_eval_with_lambda

Refactor callback setup to use lambda instead of eval
Merge pull request #17113 from igor04/helpers-highlight

Added processing of `nil` to `highlight` helper method
In render file, by default the current layout is rendered always
Merge pull request #17124 from yuktikhurana/fix_render_file_guide

In render file, by default the current layout is rendered always [ci skip]
use tr instead of gsub when possible
Merge pull request #17125 from acrogenesis/master

use tr instead of gsub when possible
Merge pull request #17088 from robin850/jruby-dev

Follow up to #16613
Add documentation on Active Job.

This adds documentation for the Active Job API. It includes
documentation on how to configure the queue_adapter, and how to create
new jobs. It adds links to the various other sections of the Active Job
documentation.
Merge pull request #17068 from mokhan/update-active-job-documentation

[ci-skip] Update Active Job API documentation