editorial work on HTML spec

Hi all, I have started to add information to the first section of each
element definition.

The additional information:

short descriptions of each of the allowed HTML attributes
tag omission details
allowed ARIA role and state and property attribute (aria-*) attribute
information.

Examples of the added content can be found in the following section
covering metadata elements:
http://www.w3.org/html/wg/drafts/html/master/document-metadata.html:

The related bug is https://www.w3.org/Bugs/Public/show_bug.cgi?id=21898


feedback welcome as always.

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Regards

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