[e] (0) Make the outline algorithm easier to edit by making it all explicit steps and breaking out the (currently still identical) steps for entering sectioning content vs sectioning roots.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=24100
Affected topics: HTML

git-svn-id: http://svn.whatwg.org/webapps@8357 340c8d12-0b0e-0410-8428-c7bf67bfef74