[1.7.x] Clarified contrib.contenttypes.generic deprecation; refs #19774.
[1.7.x] Clarified contrib.contenttypes.generic deprecation; refs #19774.

Backport of 737cd4ff3decfd3e709885d219e70d8395305965 from stable/1.7.x

jyotsna1820/django,oscaro/django,oscaro/django,jyotsna1820/django,jyotsna1820/django,jyotsna1820/django,oscaro/django,oscaro/django