Work on documentation

docs/glossary.rst

@@ -24,3 +24,18 @@
       Reference implementation of :pep:`3156`
 
       https://pypi.python.org/pypi/asyncio/
+
+   identity
+
+      Session-wide :class:`str` for identifying user.
+
+      Stored in local storage (client-side cookie or server-side storage).
+
+      Use :coroutine:`~aiohttp_session.remember` for saving *identity* (login)
+      and :coroutine:`~aiohttp_session.forget` for dropping it (logout).
+
+      *identity* is used for getting :term:`userid` and :term:`permissions`.
+
+   userid
+
+       User's ID, most likely his *login* or *email*