jquery.accessKeyLabel: Minor code and documentation clean up

* getAccessKeyPrefix parameter "ua" is optional, flag it as such.
* clarified information about accessKeyLabel and Firefox 8+.
* Renamed domElement to element. It is already specified as
  HTMLElement, no need to call it "DOM" Element as well.
* Document that it is supported to pass the same element as
  both parameters to updateTooltipOnElement.
* Move "Exposed for testing." comment to where it applies (in the
  publicly exposed copy, not the original private one).

Follows-up I211593629.

Change-Id: I3b73302ad822d7191768dde5806e9ee478327ed2
Merge "jquery.accessKeyLabel: Minor code and documentation clean up"