Document multiple issues in Botan

PR: 209595
Reported by: Sevan Janiyan <venture37@geeklan.co.uk>
Security: CVE-2015-7827
Security: CVE-2016-2849
Security: https://vuxml.FreeBSD.org/freebsd/ac0900df-31d0-11e6-8e82-002590263bf5.html
Security: CVE-2014-9742
Security: https://vuxml.FreeBSD.org/freebsd/f771880c-31cf-11e6-8e82-002590263bf5.html