added note in spec about not using headings for subtitles etc

Hi all,

as part of tidying up the obsoleting of hgroup  I have added
advice to the 5.1 spec which I will also add to the CR spec:

"Note: Do not use h1–h6 elements to markup subheadings, subtitles,
alternative titles and taglines unless they are intended to be the heading
for a new section or subsection."

commit
https://github.com/w3c/html/commit/603bf9ffc7b584198944841867996c4e9e812122

review and feedback welcome!

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Regards

SteveF
HTML 5.1 <http://www.w3.org/html/wg/drafts/html/master/>