[1.2.X] Fixed #15062 -- Documented the fact that managers must be able to be...

[1.2.X] 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.

Backport of r15220 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15221 bcc190cf-cafb-0310-a4f2-bffc1f526a37