improved _request_ctx_stack docs

docs/api.rst

@@ -301,6 +301,36 @@ Useful Internals
    instance and can be used by extensions and application code but the
    use is discouraged in general.
 
+   The following attributes are always present on each layer of the
+   stack::
+
+   `app`
+      the active Flask application.
+
+   `url_adapter`
+      the URL adapter that was used to match the request.
+
+   `request`
+      the current request object.
+
+   `session`
+      the active session object.
+
+   `g`
+      an object with all the attributes of the :data:`flask.g` object.
+
+   `flashes`
+      an internal cache for the flashed messages.
+
+   Example usage::
+
+      from flask import _request_ctx_stack
+
+      def get_session():
+          ctx = _request_ctx_stack.top
+          if ctx is not None:
+              return ctx.session
+
    .. versionchanged:: 0.4
 
    The request context is automatically popped at the end of the request