[e] (0) Mention why srcset might not always be honoured in real time.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23243
Affected topics: HTML

git-svn-id: http://svn.whatwg.org/webapps@8266 340c8d12-0b0e-0410-8428-c7bf67bfef74