security/vuxml: Document mail/mutt vulnerability

Document mail/mutt vulnerability CVE-2021-3181

PR: 252931
Submitted by: Derek Schrock <dereks@lifeofadishwasher.com>
Reported by: Derek Schrock <dereks@lifeofadishwasher.com>
Reviewed by: osa (mentor)
Approved by: osa (mentor)
Differential Revision: https://reviews.freebsd.org/D28308