Clarify the purpose of reference hashes.

The server-server specification describes a "reference hash" of an event
and how to calculate it, but is otherwise not mentioned anywhere else in
the document. This change adds a sentence to explain that they are used
for event identifiers in later room versions, which are described in
other documents.

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

changelogs/server_server/newsfragments/2159.clarification

@@ -0,0 +1 @@
+Clarify the purpose of reference hashes.

specification/server_server_api.rst

@@ -1134,7 +1134,8 @@ 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 calculated as follows.
+including content hashes. It is used for event identifiers in room versions 3 and
+higher. It is calculated as follows.
 
 1. The event is put through the redaction algorithm.