Fixed #17625 -- Mention in the management command docs that modules with an...

Fixed #17625 -- Mention in the management command docs that modules with an underscore prefix are ignored and not dissplayed in the list of commands.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17440 bcc190cf-cafb-0310-a4f2-bffc1f526a37