[e] (0) Fix prose around the <textarea> value / maxlength issues to be slightly less confusing and ambiguous. Clean up some text around MutationObservers.
Affected topics: DOM APIs, HTML

git-svn-id: http://svn.whatwg.org/webapps@8577 340c8d12-0b0e-0410-8428-c7bf67bfef74