[e] (0) Clarify that the delete dataset[foo] algorithm doesn't get invoked for values of 'foo' that don't exist.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=24096
Affected topics: HTML

git-svn-id: http://svn.whatwg.org/webapps@8359 340c8d12-0b0e-0410-8428-c7bf67bfef74