This patch adds AMI event documentation for the Cdr and CEL AMI events.

Note that while these events do share fields with each other and with other
channel related events, they do not contain all of the fields in a standard
channel snapshot, nor is the description of the fields identical. As such,
the patch opts for documentation for each field, for each event.

Review: https://reviewboard.asterisk.org/r/4350/

ASTERISK-24671 #close
Reported by: Dan Jenkins


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@430862 65c4cc65-6c06-0410-ace0-fbb531ad65f3