Add description in layout_attributes.js

Author: ayjayt

src/plots/cartesian/layout_attributes.js

@@ -23,9 +23,12 @@ var minorTickmode = {
         'If *linear*, the placement of the ticks is determined by',
         'a starting position `tick0` and a tick step `dtick`',
         '(*linear* is the default value if `tick0` and `dtick` are provided).',
-        'If *array*, the placement of the ticks is set via `tickvals`',
-        'and the tick text is `ticktext`.',
-        '(*array* is the default value if `tickvals` is provided).'
+        'If *array*, the placement of the ticks is set via `tickvals`,',
+        'which are actual values, and the tick text is `ticktext`.',
+        '(*array* is the default value if `tickvals` is provided).',
+        'If *proportional*, the placement is similiar to *array* except that',
+        '`tickvals` are fractions between 0 and 1 representing distance on',
+        'the corresponding axis.'
     ].join(' ')
 };