Documentation: Improve documentation for ExtensionStatus AMI events

Review: https://reviewboard.asterisk.org/r/4085/
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Merged revisions 426120 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@426121 f38db490-d61c-443f-a65b-d21fe96a405b