[1.8.x] Clarified deprecation of forms.forms.get_declared_fields(); refs #19617.

Backport of 89e9f81601 from master

docs/internals/deprecation.txt

@@ -238,7 +238,7 @@ details on these changes.
 * The ``use_natural_keys`` argument for ``serializers.serialize()`` will be
   removed. Use ``use_natural_foreign_keys`` instead.
 
-* ``django.forms.get_declared_fields`` will be removed.
+* Private API ``django.forms.forms.get_declared_fields()`` will be removed.
 
 * The ability to use a ``SplitDateTimeWidget`` with ``DateTimeField`` will be
   removed.

docs/releases/1.7.txt

@@ -1768,6 +1768,12 @@ available without importing ``django.contrib.sites.models`` when
 ``django.contrib.sites.shortcuts``. The old import locations will work until
 Django 1.9.
 
+``django.forms.forms.get_declared_fields()``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Django no longer uses this functional internally. Even though it's a private
+API, it'll go through the normal deprecation cycle.
+
 .. removed-features-1.7:
 
 Features removed in 1.7