Clarifications in user directory for users who share rooms tracking (#13966)

Spawned while working on [`get_users_in_room` mis-uses](https://github.com/matrix-org/synapse/pull/13958#discussion_r984074897) and thinking we could use `get_local_users_in_room` here but we can't.

From first glance, it seemed like this was only using local users from all of the `is_mine_id(user_id)` checks but I see that it does actually use remote users. Just making things a little more clear here what it does and mentions remote users so maybe that will be more obvious in the future.

changelog.d/13966.misc

@@ -0,0 +1 @@
+Refactor language in user directory `_track_user_joined_room` code to make it more clear that we use both local and remote users.

synapse/handlers/user_directory.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 import logging
-from typing import TYPE_CHECKING, Any, Dict, List, Optional
+from typing import TYPE_CHECKING, Any, Dict, List, Optional, Set, Tuple
 
 import synapse.metrics
 from synapse.api.constants import EventTypes, HistoryVisibility, JoinRules, Membership
@@ -379,7 +379,7 @@ class UserDirectoryHandler(StateDeltasHandler):
             user_id, event.content.get("displayname"), event.content.get("avatar_url")
         )
 
-    async def _track_user_joined_room(self, room_id: str, user_id: str) -> None:
+    async def _track_user_joined_room(self, room_id: str, joining_user_id: str) -> None:
         """Someone's just joined a room. Update `users_in_public_rooms` or
         `users_who_share_private_rooms` as appropriate.
 
@@ -390,32 +390,44 @@ class UserDirectoryHandler(StateDeltasHandler):
             room_id
         )
         if is_public:
-            await self.store.add_users_in_public_rooms(room_id, (user_id,))
+            await self.store.add_users_in_public_rooms(room_id, (joining_user_id,))
         else:
             users_in_room = await self.store.get_users_in_room(room_id)
             other_users_in_room = [
                 other
                 for other in users_in_room
-                if other != user_id
+                if other != joining_user_id
                 and (
+                    # We can't apply any special rules to remote users so
+                    # they're always included
                     not self.is_mine_id(other)
+                    # Check the special rules whether the local user should be
+                    # included in the user directory
                     or await self.store.should_include_local_user_in_dir(other)
                 )
             ]
-            to_insert = set()
+            updates_to_users_who_share_rooms: Set[Tuple[str, str]] = set()
 
-            # First, if they're our user then we need to update for every user
-            if self.is_mine_id(user_id):
+            # First, if the joining user is our local user then we need an
+            # update for every other user in the room.
+            if self.is_mine_id(joining_user_id):
                 for other_user_id in other_users_in_room:
-                    to_insert.add((user_id, other_user_id))
+                    updates_to_users_who_share_rooms.add(
+                        (joining_user_id, other_user_id)
+                    )
 
-            # Next we need to update for every local user in the room
+            # Next, we need an update for every other local user in the room
+            # that they now share a room with the joining user.
             for other_user_id in other_users_in_room:
                 if self.is_mine_id(other_user_id):
-                    to_insert.add((other_user_id, user_id))
+                    updates_to_users_who_share_rooms.add(
+                        (other_user_id, joining_user_id)
+                    )
 
-            if to_insert:
-                await self.store.add_users_who_share_private_room(room_id, to_insert)
+            if updates_to_users_who_share_rooms:
+                await self.store.add_users_who_share_private_room(
+                    room_id, updates_to_users_who_share_rooms
+                )
 
     async def _handle_remove_user(self, room_id: str, user_id: str) -> None:
         """Called when when someone leaves a room. The user may be local or remote.