Rephrase explanation of the purpose of reference hashes and link to the

room version spec.

Signed-off-by: Jimmy Cuadra <jimmy@jimmycuadra.com>

specification/server_server_api.rst

@@ -1134,8 +1134,9 @@ Calculating the reference hash for an event
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The *reference hash* of an event covers the essential fields of an event,
-including content hashes. It is used for event identifiers in room versions 3 and
-higher. It is calculated as follows.
+including content hashes. It is used for event identifiers in some room versions.
+See the `room version specification`_ for more information. It is calculated as
+follows.
 
 1. The event is put through the redaction algorithm.