clarify why we do this

Author: maxfischer2781

asyncstdlib/itertools.py

@@ -395,7 +395,7 @@ async def _extend_buffer(self, next_node: "_TeeNode[T]") -> None:
             if not next_node:
                 raise
         else:
-            # seek the last node that needs to be filled
+            # skip nodes that were filled in the meantime
             while next_node:
                 _, next_node = next_node  # type: ignore
             next_node[:] = next_value, []