[1.2.X] Fixed #14274 -- Added admonition about using -Wall when you run tests....

[1.2.X] Fixed #14274 -- Added admonition about using -Wall when you run tests. Thanks to Eric Holscher for the suggestion and draft, and to timo for contributions to the patch.

Backport of r14019 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14027 bcc190cf-cafb-0310-a4f2-bffc1f526a37