Make the NSM prompting have more data

(nsm-format-certificate): Include more data about the connection.
(nsm-query): Fill the text to that it looks nicer.
Make NSM warn on `high' for older protocols, and document

* doc/emacs/misc.texi (Network Security): Mention the new protocol-level
`high' NSM checks.

(nsm-check-protocol): Also warn if using SSL3 or older.