Update persist API doc

[netil]

Description

• $.isNeeded() isn't showing:
  • https://github.com/naver/egjs/blob/master/src/plugin/persist.js#L249
• Omission of persist custom event description
  • https://github.com/naver/egjs/blob/master/src/plugin/persist.js#L82

[happyhj]

$.persist.isNeeded is a nested objects methods. Will update jsdoc after reading this link. http://stackoverflow.com/questions/19230971/how-do-i-jsdoc-a-nested-objects-methods