Add description of 'none' and 'skip' flags

Author: John Soklaski

src/plots/attributes.js

@@ -74,7 +74,11 @@ module.exports = {
         flags: ['x', 'y', 'z', 'text', 'name'],
         extras: ['all', 'none', 'skip'],
         dflt: 'all',
-        description: 'Determines which trace information appear on hover.'
+        description: [
+            'Determines which trace information appear on hover.',
+            'If `none` or `skip` are set, no information is displayed upon hovering.',
+            'But, if `none` is set, click and hover events are still fired.'
+        ].join(' ')
     },
     stream: {
         token: {