DOC: Update TODO comment for a single level selection from MultiIndex

Author: Licht-T

pandas/core/groupby.py

@@ -2589,8 +2589,14 @@ def _get_grouper(obj, key=None, axis=0, level=None, sort=True,
     # validate that the passed single level is compatible with the passed
     # axis of the object
     if level is not None:
-        # TODO: These two conditions are almost same.
-        # We should combine two.
+        # TODO: These if-block and else-block are almost same.
+        # MultiIndex instance check is removable, but it seems that there are
+        # some processes only for non-MultiIndex in else-block,
+        # eg. `obj.index.name != level`. We have to consider carefully whether
+        # these are applicable for MultiIndex. Even if these are applicable,
+        # we need to check if it makes no side effect to subsequent processes
+        # on the outside of this condition.
+        # (GH 17621)
         if isinstance(group_axis, MultiIndex):
             if is_list_like(level) and len(level) == 1:
                 level = level[0]