doc: Clarifying how remainders handled

mattermost_matchmaker/matchmaking.py

@@ -63,9 +63,18 @@ def create_group(client: mattermost.MMApi, ids: Iterable[str]) -> None:
 
 
 def make_matches_for_members(client: mattermost.MMApi, members: Iterable[dict],
-                             group_size: int = 2) -> Tuple[List[str], str]:
+                             group_size: int = 2) -> Tuple[List[str], Optional[str]]:
     """
     Making matches in a channel
+
+    Side effects: A Groupchat Channel will be created for the paired groups.
+
+    A group chat would be created for the last group with less than group_size members (remainders)
+    if and only if there are more than 1 member in the remainder group. In this case, the second item
+    of the returned tuple would be None.
+
+    If the remainder group would only consist of one member, instead of creating a group for them, it is returned as
+    the second item in the tuple.
     """
 
     groups_created = []