[] (0) Make it clearer that the optional icon doesn't make sense in plain text
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=21403
Affected topics: Rendering

git-svn-id: http://svn.whatwg.org/webapps@7855 340c8d12-0b0e-0410-8428-c7bf67bfef74