[e] (0) Add some best practices notes regarding how to use metadata cues.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=24382
Affected topics: HTML, Video Text Tracks

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