Added comment [skip ci]

Author: ankane

pgvector/sparsevec.py

@@ -170,6 +170,7 @@ def _to_db_binary(cls, value: object) -> bytes | None:
 
     @classmethod
     def _from_db(cls, value: str | SparseVector | None) -> SparseVector | None:
+        # Handle conversion from low-level driver
         if value is None or isinstance(value, SparseVector):
             return value