DOC: update DataFrame.dropna's axis argument docs (pandas-dev#21652)

Author: taljaards

pandas/core/frame.py

@@ -4186,6 +4186,7 @@ def dropna(self, axis=0, how='any', thresh=None, subset=None,
 
             .. deprecated:: 0.23.0
                 Pass tuple or list to drop on multiple axes.
+                Only a single axis is allowed.
 
         how : {'any', 'all'}, default 'any'
             Determine if row or column is removed from DataFrame, when we have