Use proper smime-keys entry for S/MIME signatures using OpenSSL

Use proper smime-keys entry for S/MIME signatures using OpenSSL

bug#67931

* doc/misc/emacs-mime.texi (MML Definition):
* lisp/gnus/mml.el (mml-parse-1): Add chainfile parameter to sign tags.
* lisp/gnus/mml-smime.el (mml-smime-openssl-sign-query): Include the
additional certificates from smime-keys in MML tag generation as
chainfile parameters.
(mml-smime-openssl-sign): Forward chainfile entries from the parsed tag
alist to smime-sign-buffer.
; * lisp/gnus/smime.el (smime-sign-region): Fix typo in documentation.
; (smime-sign-buffer): Improve documentation to match smime-sign-region.