Apply suggestions from code review

Co-authored-by: Chris Broz <Chris.Broz@ucsf.edu>

src/spyglass/utils/dj_merge_tables.py

@@ -835,8 +835,13 @@ def super_delete(self, warn=True, *args, **kwargs):
         super().delete(*args, **kwargs)
 
     @classmethod
-    def extract_merge_id(cls, restriction):
+    def extract_merge_id(cls, restriction) -> Union[dict, list]:
         """Utility function to extract merge_id from a restriction
+        
+        Removes all other restricted attributes, and defaults to a
+        universal set (either empty dict or True) when there is no
+        merge_id present in the input, relying on parent func to 
+        restrict on secondary or part-parent key(s).
 
         Parameters
         ----------