Move documented but-unimplemented 'presence idle times' into a new document to...

Move documented but-unimplemented 'presence idle times' into a new document to contain such features

docs/specification-NOTHAVE.rst

+Matrix Specification NOTHAVEs
+=============================
+
+This document contains sections of the main specification that have been
+temporarily removed, because they specify intentions or aspirations that have
+in no way yet been implemented. Rather than outright-deleting them, they have
+been moved here so as to stand as an initial version for such time as they
+become extant.
+
+
+Presence
+========
+
+Idle Time
+---------
+As well as the basic ``presence`` field, the presence information can also show
+a sense of an "idle timer". This should be maintained individually by the
+user's clients, and the home server can take the highest reported time as that
+to report. When a user is offline, the home server can still report when the
+user was last seen online.

docs/specification.rst

@@ -1497,14 +1497,6 @@ in the other direction will not). This timestamp is presented via a key called
 ``last_active_ago``, which gives the relative number of miliseconds since the
 message is generated/emitted, that the user was last seen active.
 
-Idle Time
----------
-As well as the basic ``presence`` field, the presence information can also show
-a sense of an "idle timer". This should be maintained individually by the
-user's clients, and the home server can take the highest reported time as that
-to report. When a user is offline, the home server can still report when the
-user was last seen online.
-
 Transmission
 ------------
 .. NOTE::