AMI: Allow for command response documentation

Allow for responses to AMI actions/commands to be documented properly
in XML and displayed via the CLI. Response events are documented
exactly as standard AMI events are documented.

Review: https://reviewboard.asterisk.org/r/3812/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@419342 f38db490-d61c-443f-a65b-d21fe96a405b