jQuery.widget: Document 1.11's instance method for all widgets
jQuery.widget: Document that instance can be called on any element

Fixes gh-77
Dialog: Remove array and string notations for position option.
Selectmenu: Add selectmenu entry

Fixes gh-67
Menu: Menu items no longer require <a> tags.
Spinner: Document isValid() method.
Menu: Document _closeOnDocumentClick() extension point
Menu: Added _isDivider() extension point
Dialog: Get rif of "multiple types" for the position option
Menu: Fix typo
Selectmenu: Add missing instance method