[e] (0) Add more explanatory text around media element GC behaviour
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=26515
Affected topics: Video and Audio

git-svn-id: https://svn.whatwg.org/webapps@8806 340c8d12-0b0e-0410-8428-c7bf67bfef74