Document convention of using underscores in APIs

Everybody has agreed to it. Now we just have to remember to do it.

specification/client_server_api.rst

@@ -72,6 +72,15 @@ MUST be encoded as UTF-8. Clients are authenticated using opaque
 ``access_token`` strings (see `Client Authentication`_ for details), passed as a
 query string parameter on all requests.
 
+The names of the API endponts for the HTTP transport follow a convention of
+using underscores to separate words (for example ``/delete_devices``).
+
+.. NOTE::
+   There are a few historical exceptions to this rule, such as
+   ``/createRoom``. A future version of this specification will address the
+   inconsistency.
+
+
 Any errors which occur at the Matrix API level MUST return a "standard error
 response". This is a JSON object which looks like: