Clarify the meaning of "public rooms" for call invites (#2106)

changelogs/client_server/newsfragments/2106.clarification

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

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

@@ -202,11 +202,13 @@ specific user, and should be set to the Matrix user ID of that user. Invites
 without an `invitee` field are defined to be intended for any member of the
 room other than the sender of the event.
 
-Clients should consider an incoming call if they see a non-expired invite event where the `invitee` field is either
+Clients should consider an incoming call if they see a non-expired invite event
-absent or equal to their user's Matrix ID, however they should evaluate whether or not to ring based on their
+where the `invitee` field is either absent or equal to their user's Matrix ID.
-user's trust relationship with the callers and/or where the call was placed. As a starting point, it is
+They should, however, evaluate whether or not to ring based on their user's trust
-suggested that clients ignore call invites from users in public rooms. It is strongly recommended that
+relationship with the callers and/or where the call was placed. As a starting
-when clients do not ring for an incoming call invite, they still display the call invite in the room and
+point, it is RECOMMENDED that clients ignore call invites in rooms with a
+[join rule](#mroomjoin_rules) of `public`. When clients suppress ringing for an
+incoming call invite, they SHOULD still display the call invite in the room and
 annotate that it was ignored.
 
 ##### Glare