document the return of `db:test:prepare`. Closes #18136. [ci skip]

This is a doc follow-up to #17739.

/cc @zzak
s/are edited and rerun/required modifications [ci skip]
Remove this line since we are using Rake to run our tests [ci skip]
I'm sorry but I would rather not link to a 7 year old blog post [ci skip]
Add subheadings for these sections [ci skip]
Specify test_order to not show deprecation message

Running the following test was showing the below deprecation message:
> cd raities
> ruby -w -Itest test/generators/generator_test.rb

    DEPRECATION WARNING: You did not specify a value for the
    configuration option `active_support.test_order`. In
    Rails 5, the default value...