Merge pull request matplotlib#3184 from tacaswell/get_window_extent_doc

DOC : added warning to doc of get_window_extent

Author: dmcdougall

Diff for: lib/matplotlib/artist.py

@@ -193,6 +193,14 @@ def get_window_extent(self, renderer):
         Subclasses should override for inclusion in the bounding box
         "tight" calculation. Default is to return an empty bounding
         box at 0, 0.
+
+        Be careful when using this function, the results will not update
+        if the artist window extent of the artist changes.  The extent
+        can change due to any changes in the transform stack, such as
+        changing the axes limits, the figure size, or the canvas used
+        (as is done when saving a figure).  This can lead to unexpected
+        behavior where interactive figures will look fine on the screen,
+        but will save incorrectly.
         """
         return Bbox([[0, 0], [0, 0]])
 
@@ -644,7 +652,7 @@ def set_clip_on(self, b):
         """
         Set whether artist uses clipping.
 
-        When False artists will be visible out side of the axes which 
+        When False artists will be visible out side of the axes which
         can lead to unexpected results.
 
         ACCEPTS: [True | False]