fix: add UserIsManagerError to the Raises clause of group_repo.remove…

…_member() docstring

src/dioptra/restapi/db/repository/groups.py

@@ -487,14 +487,16 @@ def remove_member(self, group: Group, user: User) -> None:
             EntityDeletedError: if the group or user is deleted
             GroupNeedsAUserError: if removal would leave the group userless
             UserNeedsAGroupError: if removal would leave the user groupless
+            UserIsManagerError: if there are sufficient members and memberships
+                to remove the user, but the user is a manager of the group
         """
 
         # Consistency rules:
         # - A group must have at least one member
+        # - A group must have at least one manager
+        # - A group manager must be a member
         # - A user must be in at least one group
 
-        # TODO: ensure the member being removed is not also manager
-
         assert_group_exists(self.session, group, DeletionPolicy.NOT_DELETED)
         assert_user_exists(self.session, user, DeletionPolicy.NOT_DELETED)