This is a doc follow-up to #17739.

/cc @zzak

Fixes #18122

Fix inaccurate docs in active_model errors [ci skip]

Add note about Ajax and CSRF-Token [ci skip]

Clarify ActiveRecord testing guidelines [ci skip]

`bin/rake` does not exist in plugin directory [ci skip]

[ci skip]

[ci skip]

The default value for the argument `message` in
`ActiveModel::Errors#add` has a new behavior
since ca99ab24.

Before
  person.errors.add(:name, nil)
  # => ["is invalid"]

After
  person.errors.add(:name, nil)
  # => [nil]

Fix typo in nodoc [ci skip]

Remove unused Sheep fixture from AV

The `Sheep` fixture was added in eb23754e when moving template tests
from actionpack to actionview, but it's not actually used in ActionView tests.

The `Sheep` fixture is only used to test `uncountable` in ActiveModel tests,
and is already defined in activemodel/test/models/sheep.rb

Remove extra bullet in README.md [ci skip]

Validation test fixes

Add 'require' for Hash#with_indifferent_access to active_job/arguments.rb

1. Specify that you need to create the test databases, and that no special
   Rails command needs to be run to do that.
2. Although the underscore style of `rake test_mysql` works, make the
   documentation of running the tests in RUNNING_UNIT_TESTS.rdoc
   consistent with the "Contributing..." guide.
3. Promote "Testing Active Record" to not be a subsection of
   "Running a Single Test," since it doesn't make sense as a subsection
   of that.

Testing guide grammar fix

[skip ci]

ActiveJob::Arguments uses Hash#with_indifferent_access.
But, activejob gem does not require Hash extension library.
When we use activejob as standalone,
we need extra require statement. This is unhandy.

This commit fixes it.

- Changed test to verify complete message instead of verifying if message contains text.

[ci skip]

Follow-up to d9710212 [skip ci]