Backport PR #54728 on branch 2.1.x (Docs: Update doc string to not reference deprecated function fillna for ffill) (#54740)

Backport PR #54728: Docs: Update doc string to not reference deprecated function fillna for ffill

Co-authored-by: Jessica Greene <jessica0greene@gmail.com>

Author: meeseeksmachine

pandas/core/generic.py

@@ -7348,7 +7348,7 @@ def ffill(
         downcast: dict | None | lib.NoDefault = lib.no_default,
     ) -> Self | None:
         """
-        Synonym for :meth:`DataFrame.fillna` with ``method='ffill'``.
+        Fill NA/NaN values by propagating the last valid observation to next valid.
 
         Returns
         -------