Clarify pivot_table fill_value description (pandas-dev#32618)

Author: taljaards

pandas/core/frame.py

@@ -5903,7 +5903,8 @@ def pivot(self, index=None, columns=None, values=None) -> "DataFrame":
             If dict is passed, the key is column to aggregate and value
             is function or list of functions.
         fill_value : scalar, default None
-            Value to replace missing values with.
+            Value to replace missing values with (in the resulting pivot table,
+            after aggregation).
         margins : bool, default False
             Add all row / columns (e.g. for subtotal / grand totals).
         dropna : bool, default True