Editorial suggestions from Benjamin Kaduk's IESG review

[kaduk]

Mostly minor stuff, markdown and grammar nits and the like.

The one (?) exception is changing the presentation language for an opaque signature<> entry to have a minimum length of 1 instead of 0. There are two opaque signature<> elements in the document, and the other one already had this change applied due to a previous review of mine, so I claim it still is effectively "minor".

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