Clarify the meaning of "public rooms" for presence (#2108)

changelogs/client_server/newsfragments/2108.clarification

@@ -0,0 +1 @@
+"Public" rooms with respect to presence are defined through their join rule.

content/client-server-api/modules/presence.md

@@ -68,5 +68,7 @@ will cause the server to automatically set their presence to `online`.
 
 #### Security considerations
 
-Presence information is shared with all users who share a room with the
-target user. In large public rooms this could be undesirable.
+Presence information is published to all users who share a room with the
+target user. If the target user is a member of a room with a `public`
+[join rule](#mroomjoin_rules), any other user in the federation is
+able to gain access to the target user's presence. This could be undesirable.