Extend beforeShowMonth documentation

Extend beforeShowMonth documentation

Author: tpeter1985

docs/options.rst

@@ -37,7 +37,16 @@ beforeShowMonth
 
 Function(Date).  Default: $.noop
 
-A function that takes a date as a parameter and returns a boolean indicating whether or not this month is selectable
+A function that takes a date as a parameter and returns one of the following values:
+
+ * undefined to have no effect
+ * A Boolean, indicating whether or not this month is selectable
+ * A String representing additional CSS classes to apply to the month's cell
+ * An object with the following properties:
+
+   * ``enabled``: same as the Boolean value above
+   * ``classes``: same as the String value above
+   * ``tooltip``: a tooltip to apply to this date, via the ``title`` HTML attribute
 
 
 beforeShowYear