Update sticky_edges docstring to new behavior.

The behavior was tweaked in 0911f8a.

Author: anntzer

lib/matplotlib/artist.py

@@ -1119,6 +1119,11 @@ def sticky_edges(self):
         where one usually expects no margin on the bottom edge (0) of the
         histogram.
 
+        Moreover, margin expansion "bumps" against sticky edges and cannot
+        cross them.  For example, if the upper data limit is 1.0, the upper
+        view limit computed by simple margin application is 1.2, but there is a
+        sticky edge at 1.1, then the actual upper view limit will be 1.1.
+
         This attribute cannot be assigned to; however, the ``x`` and ``y``
         lists can be modified in place as needed.