Mention MTA-STS in DANE context; nit fixes

Mention MTA-STS in DANE context; nit fixes

Did an audit of text changed since commit 6aa6fc9c5 to look for issues
which stood out, fixed those.  Spelling mistakes, markup issues, minor
grammatical infelicities.

The public/private CA stuff in the DANE text might push people away from
public CAs, but the existence of MTA-STS means that one of those is
probably the best choice.  Mention what exim.org does, to provide
slightly firmer guidance without pressure.

List the `dkim_hash` values, `sha512` appears to be new since that text
was last touched.