Clarify the values returned by computedTiming

As reported here:
http://lists.w3.org/Archives/Public/public-fx/2014OctDec/0120.html

(Also, fix up a number of links to attributes throughout the spec)

Generated from:

commit 5d43856e957bffd9dc1ec28b286197d9ac8ec3d9
Author: Brian Birtles <birtles@gmail.com>
Date:   Tue Dec 16 16:26:01 2014 +0900

    Clarify the values returned by computedTiming

    As reported here:
    http://lists.w3.org/Archives/Public/public-fx/2014OctDec/0120.html

    (Also, fix up a number of links to attributes throughout the spec)