Merge pull request matplotlib#2929 from fariza/clip-on-doc-warning

clip_on documentation note/warning

Author: tacaswell

lib/matplotlib/artist.py

@@ -644,6 +644,9 @@ def set_clip_on(self, b):
         """
         Set whether artist uses clipping.
 
+        When False artists will be visible out side of the axes which 
+        can lead to unexpected results.
+
         ACCEPTS: [True | False]
         """
         self._clipon = b