[giow] (1) Clarify TextMetrics.emHeightAscent and emHeightDescent in the multifont case.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=24723
Affected topics: Canvas

git-svn-id: http://svn.whatwg.org/webapps@8486 340c8d12-0b0e-0410-8428-c7bf67bfef74