Improve docs

docs/glossary.rst

@@ -47,6 +47,13 @@
 
       *identity* is used for getting :term:`userid` and :term:`permissions`.
 
+   permission
+
+      Permission required for access to resource.
+
+      Permissions are just strings, and they have no required
+      composition: you can name permissions whatever you like.
+
    userid
 
        User's ID, most likely his *login* or *email*

docs/reference.rst

@@ -78,7 +78,7 @@ Public API functions
 
    :param request: :class:`aiohttp.web.Request` object.
 
-   :param str permission: requested permission.
+   :param str permission: requested :term:`permission`.
 
    :param context: additional object may be passed into
                    :meth:`AbstractAuthorizationPolicy.permission`