Make minor edits and add comments

Author: howsiwei

pandas/core/indexing.py

@@ -202,11 +202,13 @@ def _get_setitem_indexer(self, key):
     def __setitem__(self, key, value):
         if isinstance(key, tuple):
             key = tuple(com.apply_if_callable(x, self.obj) for x in key)
+            # if `key` is an index to multiple columns by name, add each column
+            # if it does not already exist
             if (
-                self.name == "loc"
-                and len(key) > 1
-                and is_list_like_indexer(key[1])
-                and not isinstance(key[1], tuple)
+                self.name == "loc"  # column is indexed by name
+                and len(key) >= 2  # key is at least 2-dimensional
+                and is_list_like_indexer(key[1])  # key indexes multiple columns
+                and not isinstance(self.obj._get_axis(1), ABCMultiIndex)
                 and not com.is_bool_indexer(key[1])
                 and all(is_hashable(k) for k in key[1])
             ):