Update docstring on Handlers.

To indicate it is deprecated.

synapse/handlers/__init__.py

@@ -31,10 +31,22 @@ from .search import SearchHandler
 
 class Handlers(object):
 
-    """ A collection of all the event handlers.
+    """
+    Deprecated.
 
-    There's no need to lazily create these; we'll just make them all eagerly
-    at construction time.
+    At some point most of the classes whose name ended "Handler" were
+    accessed through this class.
+
+    However this makes it painful to unit test the handlers and to run cut
+    down versions of synapse that only use specific handlers because using a
+    single handler required creating all of the handlers. So some of the
+    handlers have been lifted out of the Handlers object and are now accessed
+    directly through the homeserver object itself.
+
+    Any new handlers should follow the new pattern of being accessed through
+    the homeserver object and should not be added to the Handlers object.
+
+    The remaining handlers should be moved out of the handlers object.
     """
 
     def __init__(self, hs):