Documented some Oracle-specific test settings.

Patch from aaugustin that should help people with specific testing
requirements for Oracle set things up in their preferred way. The
settings have always existed, but now they are documented.

Fixes #16478.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16646 bcc190cf-cafb-0310-a4f2-bffc1f526a37