Backport PR #39978: DOC: fix template substitution in pad/bfill docstrings (#40017)

Co-authored-by: Dylan Percy <[email protected]>

Author: meeseeksmachine

pandas/core/generic.py

@@ -6438,6 +6438,7 @@ def fillna(
             return result.__finalize__(self, method="fillna")
 
     @final
+    @doc(klass=_shared_doc_kwargs["klass"])
     def ffill(
         self: FrameOrSeries,
         axis=None,
@@ -6460,6 +6461,7 @@ def ffill(
     pad = ffill
 
     @final
+    @doc(klass=_shared_doc_kwargs["klass"])
     def bfill(
         self: FrameOrSeries,
         axis=None,