Update misleading documentation ` user_directory.search_all_users ` (#14818)

Fixes #13852

changelog.d/14818.doc

+Updated documentation in configuration manual for `user_directory.search_all_users`.
\ No newline at end of file

docs/usage/configuration/config_documentation.md

@@ -3472,8 +3472,8 @@ This setting defines options related to the user directory.
 This option has the following sub-options:
 * `enabled`:  Defines whether users can search the user directory. If false then
    empty responses are returned to all queries. Defaults to true.
-* `search_all_users`: Defines whether to search all users visible to your HS when searching
-   the user directory. If false, search results will only contain users
+* `search_all_users`: Defines whether to search all users visible to your HS at the time the search is performed. If set to true, will return all users who share a room with the user from the homeserver.
+   If false, search results will only contain users
     visible in public rooms and users sharing a room with the requester.
     Defaults to false.