ARI: Improvements to body parameters documentation

ARI: Improvements to body parameters documentation

The variables body parameter under the originate and originate with id
operations of the channel resource showed invalid JSON in its description.

The variables body parameter under the userEvent operation of the event
resource made no mention that the custom key/value pairs should be wrapped
in a variables key in order to be added to the custom user event.

ASTERISK-23975 #close

Review: https://reviewboard.asterisk.org/r/3692/
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Merged revisions 417878 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@417879 65c4cc65-6c06-0410-ace0-fbb531ad65f3