Clarify that the common identifier format can be broken

We already reference in the Event IDs section that the format depends on the room version, so we just need to link there.

Fixes https://github.com/matrix-org/matrix-doc/issues/2103

specification/appendices/identifier_grammar.rst

@@ -88,8 +88,8 @@ Common Identifier Format
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 The Matrix protocol uses a common format to assign unique identifiers to a
-number of entities, including users, events and rooms. Each identifier takes
+number of entities, including users, events and rooms. Each identifier typically
-the form::
+takes the form::
 
   &localpart:domain
 
@@ -106,7 +106,9 @@ The sigil characters are as follows:
 * ``#``: Room alias
 
 The precise grammar defining the allowable format of an identifier depends on
-the type of identifier.
+the type of identifier. For example, event IDs can be represented without a
+``domain`` component under some conditions - see the `Event IDs <#room-ids-and-event-ids>`_
+section below for more information.
 
 User Identifiers
 ++++++++++++++++