Add note about Ajax and CSRF-Token [ci skip]
Clarify ActiveRecord testing guidelines [ci skip]
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.
`bin/rake` does not exist in plugin directory [ci skip]
Merge pull request #18131 from y-yagi/fix_command_in_plugins_guide

`bin/rake` does not exist in plugin directory [ci skip]
Merge pull request #18125 from codeodor/master

Clarify ActiveRecord testing guidelines [ci skip]
Merge pull request #18105 from andreynering/guides-csrf

Add note about Ajax and CSRF-Token [ci skip]
s/a unobtrusive/an unobtrusive [ci skip]