Merge pull request matplotlib#4587 from tacaswell/doc_contour_autoN

DOC: clairify auto-level behavior

Author: efiring

lib/matplotlib/contour.py

@@ -1667,7 +1667,7 @@ def _initialize_x_y(self, z):
           contour(Z,N)
           contour(X,Y,Z,N)
 
-        contour *N* automatically-chosen levels.
+        contour up to *N* automatically-chosen levels.
 
         ::