[1.5.X] Fixed #19384 -- Documented the behavior of custom managers on abstract models.

This documents the behavior introduced by cc337a74, which is BACKWARDS
INCOMPATIBLE for any attempt to invoke a method on a manager using the
abstract class as the calling class (e.g., AbstractBase.objects.do_something())

Thanks to mhsparks for the report.

Backport of 1b646e65 from master.