Added import locations to contrib.postgres aggregates and validators docs.

docs/ref/contrib/postgres/aggregates.txt

@@ -5,7 +5,8 @@ PostgreSQL specific aggregation functions
 .. module:: django.contrib.postgres.aggregates
    :synopsis: PostgreSQL specific aggregation functions
 
-These functions are described in more detail in the `PostgreSQL docs
+These functions are available from the ``django.contrib.postgres.aggregates``
+module. They are described in more detail in the `PostgreSQL docs
 <https://www.postgresql.org/docs/current/static/functions-aggregate.html>`_.
 
 .. note::

docs/ref/contrib/postgres/validators.txt

@@ -4,6 +4,9 @@ Validators
 
 .. module:: django.contrib.postgres.validators
 
+These validators are available from the ``django.contrib.postgres.validators``
+module.
+
 ``KeysValidator``
 =================