Fixed #14861 -- Added an admonition about the potential for circular imports...

Fixed #14861 -- Added an admonition about the potential for circular imports with custom logging handlers (which produces an extremely confusing error message) to the logging docs. Thanks to donspaulding for the report and adamv for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15449 bcc190cf-cafb-0310-a4f2-bffc1f526a37