DOC: Update pct_change documentation (#51262)

* DOC: Update pct_change documentation

Related to SO question: https://stackoverflow.com/questions/75395345/how-to-avoid-bfill-or-ffill-when-calculating-pct-change-with-nans

* DOC: Update pct_change documentation

Author: corralien

pandas/core/generic.py

@@ -10797,7 +10797,7 @@ def pct_change(
         ----------
         periods : int, default 1
             Periods to shift for forming percent change.
-        fill_method : str, default 'pad'
+        fill_method : {'backfill', 'bfill', 'pad', 'ffill', None}, default 'pad'
             How to handle NAs **before** computing percent changes.
         limit : int, default None
             The number of consecutive NAs to fill before stopping.