Fixed #15062 -- Documented the fact that managers must be able to be shallow copied. Thanks to Ian Clelland for the report, and Łukasz Rekucki for the help diagnosing the problem.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15220 bcc190cf-cafb-0310-a4f2-bffc1f526a37