Merge pull request matplotlib#3877 from nickystringer/unknown_properties_docs

Improved the marker docs

Author: pelson

lib/matplotlib/lines.py

@@ -971,6 +971,8 @@ def set_marker(self, marker):
         """
         Set the line marker
 
+        ACCEPTS: :mod:`A valid marker style <matplotlib.markers>`
+
         Parameters
         -----------