Make more TLS checks trigger on the default `medium' level

Make more TLS checks trigger on the default `medium' level

* doc/emacs/misc.texi (Network Security): Update the doc to say
what's on the different levels.

* lisp/net/nsm.el (nsm-protocol-check--intermediary-sha1): Check
intermediary certificates for SHA1.
(nsm-protocol-check--3des): Check for 3DES ciphers.
(network-security-protocol-checks): Put most of the checks on
`medium'.