Add docstring clarification.

svn path=/trunk/matplotlib/; revision=6572

Author: mdboom

lib/matplotlib/artist.py

@@ -340,6 +340,8 @@ def get_snap(self):
 
           * None: (auto) If the path contains only rectilinear line
             segments, round to the nearest pixel center
+
+        Only supported by the Agg backends.
         """
         return self._snap
 
@@ -353,6 +355,8 @@ def set_snap(self, snap):
 
           * None: (auto) If the path contains only rectilinear line
             segments, round to the nearest pixel center
+
+        Only supported by the Agg backends.
         """
         self._snap = snap