DOC: update applymap docstring in pandas/core/frame.py (pandas-dev#24612) (pandas-dev#24633)

Author: nixphix

pandas/core/frame.py

@@ -6501,6 +6501,14 @@ def applymap(self, func):
         --------
         DataFrame.apply : Apply a function along input axis of DataFrame.
 
+        Notes
+        -----
+        In the current implementation applymap calls `func` twice on the
+        first column/row to decide whether it can take a fast or slow
+        code path. This can lead to unexpected behavior if `func` has
+        side-effects, as they will take effect twice for the first
+        column/row.
+
         Examples
         --------
         >>> df = pd.DataFrame([[1, 2.12], [3.356, 4.567]])