Backport PR pandas-dev#39139: BUG: Placeholders not being filled on docstrings

Author: MicaelJarniac

pandas/core/generic.py

@@ -10889,8 +10889,10 @@ def all(self, axis=0, bool_only=None, skipna=True, level=None, **kwargs):
         # [assignment]
         cls.all = all  # type: ignore[assignment]
 
+        # error: Argument 1 to "doc" has incompatible type "Optional[str]"; expected
+        # "Union[str, Callable[..., Any]]"
         @doc(
-            NDFrame.mad,
+            NDFrame.mad.__doc__,  # type: ignore[arg-type]
             desc="Return the mean absolute deviation of the values "
             "over the requested axis.",
             name1=name1,