DOC: add code comment

Author: tacaswell

lib/matplotlib/cbook/__init__.py

@@ -1991,6 +1991,7 @@ def _warn_external(message, category=None):
     frame = sys._getframe()
     for stacklevel in itertools.count(1):  # lgtm[py/unused-loop-variable]
         if frame is None:
+            # when called in embedded context may hit frame is None
             break
         if not re.match(r"\A(matplotlib|mpl_toolkits)(\Z|\.)",
                         # Work around sphinx-gallery not setting __name__.