security_events: Add AMI documentation; output optional fields

This patch adds documentation for the Security Events that are emited over
AMI. It also notes these events in the UPGRADE/CHANGES file.
........

Merged revisions 407589 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@407602 f38db490-d61c-443f-a65b-d21fe96a405b