Add missing docstring,.

ops/model.py

@@ -1700,6 +1700,7 @@ def __getitem__(self, key: str) -> str:
 
     # This is provided by Mapping, but we need to make the return type more explicit.
     def get(self, key: str, default: Optional[str] = None) -> Optional[str]:
+        """Return the vaule for key if key is in the data, else default."""
         try:
             return self.__getitem__(key)
         except KeyError: