update comment

Author: PCerles

pandas/core/reshape/merge.py

@@ -830,7 +830,7 @@ def _maybe_add_join_keys(self, result, left_indexer, right_indexer):
                     rvals = algos.take_1d(take_right, right_indexer, fill_value=rfill)
 
                 # if we have an all missing left_indexer
-                # make sure to just use the right values
+                # make sure to just use the right values or vice-versa
                 mask_left = left_indexer == -1
                 mask_right = right_indexer == -1
                 if mask_left.all():