simplify instructions for regenerating user_dir

docs/user_directory.md

+User Directory API Implementation
+=================================
+
+The user directory is currently maintained based on the 'visible' users
+on this particular server - i.e. ones which your account shares a room with, or
+who are present in a publicly viewable room present on the server.
+
+The directory info is stored in various tables, which can (typically after
+DB corruption) get stale or out of sync.  If this happens, for now the
+quickest solution to fix it is:
+
+```
+UPDATE user_directory_stream_pos SET stream_id = NULL;
+```
+
+and restart the synapse, which should then start a background task to
+flush the current tables and regenerate the directory.