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]

[ci skip]

Follow-up to d9710212 [skip ci]

[ci skip]

[ci skip]

[ci skip]

YAML's behavior in this guide. [ci skip]

this should be `:nodoc:` in order to be parsed.

Improve grammar in configuring serve_static_files guide [ci skip]

[ci skip]

Update description for `start` parameter.

The term 'constant autoloading' can be mistakenly
construed as 'constantly autoloading'.

Replace guides.ror.org/v4.2.0 with guides.ror.org

No need to point links to http://guides.rubyonrails.org/v4.2.0/... now
that the Ruby on Rails guides point to 4.2.0.

Fix link in autoloading guide

[ci skip]

Add link for Domain-Specific Language [ci skip]

Make bin/setup portable

Add nodoc to some constants [skip ci]

Allow static asset serving from env variable (enhanced!)
Conflicts:
	railties/CHANGELOG.md

`db:schema:load` and `db:structure:load` do not purge the database