Add docstring for MultiIndex.fillna (fixes pandas-dev#18018)

Author: Ghasem Naddaf

pandas/core/indexes/multi.py

@@ -811,7 +811,9 @@ def duplicated(self, keep='first'):
 
     @Appender(ibase._index_shared_docs['fillna'])
     def fillna(self, value=None, downcast=None):
-        # isna is not implemented for MultiIndex
+        """
+        isna is not implemented for MultiIndex
+        """
         raise NotImplementedError('isna is not defined for MultiIndex')
 
     @Appender(_index_shared_docs['dropna'])