Always add local users to the user directory (#10796)

It's a simplification, but one that'll help make the user directory logic easier
to follow with the other changes upcoming. It's not strictly required for those
changes, but this will help simplify the resulting logic that listens for
`m.room.member` events and generally make the logic easier to follow.

This means the config option `search_all_users` ends up controlling the
search query only, and not the data we store. The cost of doing so is an
extra row in the `user_directory` and `user_directory_search` tables for
each local user which

- belongs to no public rooms
- belongs to no private rooms of size ≥ 2

I think the cost of this will be marginal (since they'll already have entries
 in `users` and `profiles` anyway).

As a small upside, a homeserver whose directory was built with this
change can toggle `search_all_users` without having to rebuild their
directory.

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

synapse/config/user_directory.py

@@ -45,12 +45,16 @@ class UserDirectoryConfig(Config):
             #enabled: false
 
             # Defines whether to search all users visible to your HS when searching
-            # the user directory, rather than limiting to users visible in public
-            # rooms. Defaults to false.
+            # the user directory. If false, search results will only contain users
+            # visible in public rooms and users sharing a room with the requester.
+            # Defaults to false.
             #
-            # If you set it true, you'll have to rebuild the user_directory search
-            # indexes, see:
-            # https://matrix-org.github.io/synapse/latest/user_directory.html
+            # NB. If you set this to true, and the last time the user_directory search
+            # indexes were (re)built was before Synapse 1.44, you'll have to
+            # rebuild the indexes in order to search through all known users.
+            # These indexes are built the first time Synapse starts; admins can
+            # manually trigger a rebuild following the instructions at
+            #     https://matrix-org.github.io/synapse/latest/user_directory.html
             #
             # Uncomment to return search results containing all known users, even if that
             # user does not share a room with the requester.