Clarification of contenttypes docs

[psmith]

I found the paragraph about using a GenericForeignKey in a GenericRelation to be somewhat confusing at first read, and the example line of code explicitly sets the content_type_field, even though the example of Comment uses a default value for content_type_field.

[psmith]

Patch against docs/ref/contenttypes.txt

[Tim Graham]

further clarification based on existing patch

[Tim Graham]

I tried to clarify the wording a bit further, but I'll leave it up to the committer to decide which version he or she thinks is most clear. Here's the final text I came up with:

Note that if the model in a GenericRelation uses a non-default value for
ct_field or fk_field in its GenericForeignKey (e.g. the
django.contrib.comments app uses ct_field="object_pk"), you'll need to 
set content_type_field and/or object_id_field in the GenericRelation to
match the ct_field and fk_field, respectively, in the GenericForeignKey

[Russell Keith-Magee]

(In [13554]) Fixed #11047 -- Clarified the explanation of arguments to GenericForeignKey. Thanks to psmith and timo for their work on the patch.

[Russell Keith-Magee]

(In [13561]) [1.2.X] Fixed #11047 -- Clarified the explanation of arguments to GenericForeignKey. Thanks to psmith and timo for their work on the patch.

Backport of r13554 from trunk.