Clarify the meaning of "public rooms" for user directory queries (#2102)

changelogs/client_server/newsfragments/2102.clarification

@@ -0,0 +1 @@
+"Public" rooms in user directory queries are defined through their join rule and history visibility.

data/api/client-server/users.yaml

@@ -20,10 +20,17 @@ paths:
     post:
       summary: Searches the user directory.
       description: |-
-        Performs a search for users. The homeserver may
+        Performs a search for users. The homeserver may determine which
-        determine which subset of users are searched, however the homeserver
+        subset of users are searched. However, the homeserver MUST at a
-        MUST at a minimum consider the users the requesting user shares a
+        minimum consider users who are visible to the requester based
-        room with and those who reside in public rooms (known to the homeserver).
+        on their membership in rooms known to the homeserver. This means:
+
+        -   users that share a room with the requesting user
+        -   users who are joined to rooms known to the homeserver that have a
+            `public` [join rule](#mroomjoin_rules)
+        -   users who are joined to rooms known to the homeserver that have a
+            `world_readable` [history visibility](#room-history-visibility)
+        
         The search MUST consider local users to the homeserver, and SHOULD
         query remote users as part of the search.