[2.1.x] Refs #29593, #26891 -- Doc'd RegisterLookupMixin.get_lookups().

Backport of a67ebcb1cb from master

docs/ref/models/lookups.txt

@@ -57,6 +57,11 @@ register lookups on itself. The two prominent examples are
         and checks if any has a registered lookup named ``lookup_name``, returning
         the first match.
 
+    .. method:: get_lookups()
+
+        Returns a dictionary of each lookup name registered in the class mapped
+        to the :class:`Lookup` class.
+
     .. method:: get_transform(transform_name)
 
         Returns a :class:`Transform` named ``transform_name``. The default