Document group.membership service's getGroupMembers() method.

docs/development/module/services.md

@@ -81,6 +81,9 @@ array of asset entities.
 `getGroupAssignmentLog($asset)` - Find the latest group assignment log that
 references an asset. Returns a log entity, or `NULL` if no logs were found.
 
+`getGroupMembers($group, $recurse)` - Get assets that are members of a group,
+optionally recursing into child groups.
+
 **Example usage:**
 
 ```php