Merge pull request matplotlib#3879 from nickystringer/unknown_properties_docs_2

Change mark_every docs

Author: pelson

lib/matplotlib/lines.py

@@ -463,6 +463,9 @@ def set_markevery(self, every):
 
         e.g., if `every=5`, every 5-th marker will be plotted.
 
+        ACCEPTS: [None | int | length-2 tuple of int | slice |
+        list/array of int | float | length-2 tuple of float]
+
         Parameters
         ----------
         every: None | int | length-2 tuple of int | slice | list/array of int |