DOC: clarify idxmax behaviour issue pandas-dev#18206

Author: Ghasem Naddaf

pandas/core/frame.py

@@ -5812,7 +5812,7 @@ def idxmax(self, axis=0, skipna=True):
             0 or 'index' for row-wise, 1 or 'columns' for column-wise
         skipna : boolean, default True
             Exclude NA/null values. If an entire row/column is NA, the result
-            will be first index.
+            will be NA.
 
         Returns
         -------

pandas/core/series.py

@@ -1336,7 +1336,8 @@ def idxmax(self, axis=None, skipna=True, *args, **kwargs):
         Parameters
         ----------
         skipna : boolean, default True
-            Exclude NA/null values
+            Exclude NA/null values. If an entire row/column is NA, the result
+            will be NA.
 
         Returns
         -------