Added uuid support for new session serialization and documented it

docs/api.rst

@@ -224,6 +224,18 @@ implementation that Flask is using.
 .. autoclass:: SessionMixin
    :members:
 
+.. autodata:: session_json_serializer
+
+   This object provides dumping and loading methods similar to simplejson
+   but it also tags certain builtin Python objects that commonly appear in
+   sessions.  Currently the following extended values are supported in
+   the JSON it dumps:
+
+   -    :class:`~markupsafe.Markup` objects
+   -    :class:`~uuid.UUID` objects
+   -    :class:`~datetime.datetime` objects
+   -   :class:`tuple`\s
+
 .. admonition:: Notice
 
    The ``PERMANENT_SESSION_LIFETIME`` config key can also be an integer