Rewrite `activerecord/RUNNING_UNIT_TESTS`

`RUNNING_UNIT_TESTS` is written in an unclear and confusing manner. This commit
attempts to correct that.

Thanks to zzak and vipulnsward for feedback.
Merge pull request #10601 from Ichimonji10/fix-docs

Rewrite  `activerecord/RUNNING_UNIT_TESTS.rdoc`