[selection-api] Add a "Selection interface" section (#29)

Most specs have a dedicated section, for important interfaces, so that they can be linked easily:
https://dom.spec.whatwg.org/#interface-document
http://dev.w3.org/csswg/cssom/#the-stylesheet-interface

This would be nice for the Selection API spec as well, at first glance I thought the Selection interface wasn't actually in the spec. Maybe just rename "Definition" to "The Selection Interface" or something.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/selection-api/issues/29