restrict openapi spec based on sql grants

docs/references/api/openapi.rst

@@ -9,6 +9,8 @@ PostgREST automatically serves a full `OpenAPI <https://www.openapis.org/>`_ des
 
   By default, this output depends on the permissions of the role that is contained in the JWT role claim (or the :ref:`db-anon-role` if no JWT is sent). If you need to show all the endpoints disregarding the role's permissions, set the :ref:`openapi-mode` config to :code:`ignore-privileges`.
 
+  When following privileges, the output reflects both the granted HTTP methods and columns: a relation with only ``SELECT`` will only expose ``GET``, a relation with only ``INSERT`` will only expose ``POST``, and column-level grants limit the columns shown on the table definitions and row filters.
+
 For extra customization, the OpenAPI output contains a "description" field for every `SQL comment <https://www.postgresql.org/docs/current/sql-comment.html>`_ on any database object. For instance,
 
 .. code-block:: postgres