[2.1.x] Fixed #29317 -- Doc'd filter argument in contrib.postgres aggregates.

Backport of 926fa7116f from master.

docs/ref/contrib/postgres/aggregates.txt

@@ -16,6 +16,11 @@ These functions are described in more detail in the `PostgreSQL docs
         >>> SomeModel.objects.aggregate(arr=ArrayAgg('somefield'))
         {'arr': [0, 1, 2]}
 
+.. admonition:: Common aggregate options
+
+    All aggregates have the :ref:`filter <aggregate-filter>` keyword
+    argument.
+
 General-purpose aggregation functions
 =====================================
 

docs/ref/models/querysets.txt

@@ -3262,6 +3262,8 @@ of the return value
     ``output_field`` if all fields are of the same type. Otherwise, you
     must provide the ``output_field`` yourself.
 
+.. _aggregate-filter:
+
 ``filter``
 ~~~~~~~~~~