update doc for `$.Button`
update doc for `$.Button`

relates to #1080

 On branch issue-1080
 Changes to be committed:
	modified:   src/button.js

seanich/openseadragon,openseadragon/openseadragon,seanich/openseadragon,openseadragon/openseadragon,LarissaSmith/openseadragon,LarissaSmith/openseadragon