Renamed descriptor classes for related objects.

The old names were downright confusing. Some seemed to mean the opposite
of what the class actually did.

The new names follow a consistent nomenclature:

    (Forward|Reverse)(ManyToOne|OneToOne|ManyToMany)Descriptor.

I mentioned combinations that do not exist in the docstring in order to
help people who would search for them in the code base.

tests/foreign_object/models/article.py

@@ -1,11 +1,10 @@
 from django.db import models
-from django.db.models.fields.related import \
-    ReverseSingleRelatedObjectDescriptor
+from django.db.models.fields.related import ForwardManyToOneDescriptor
 from django.utils.encoding import python_2_unicode_compatible
 from django.utils.translation import get_language
 
 
-class ArticleTranslationDescriptor(ReverseSingleRelatedObjectDescriptor):
+class ArticleTranslationDescriptor(ForwardManyToOneDescriptor):
     """
     The set of articletranslation should not set any local fields.
     """