[e] (0) Try to clarify the ping='' header requirements.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=24086
Affected topics: HTML

git-svn-id: http://svn.whatwg.org/webapps@8353 340c8d12-0b0e-0410-8428-c7bf67bfef74