documentation: Document built-in system and channel vars

Documentation for built-in special system and channel
vars is currently outdated, and updating is a manual
process since there is no XML documentation for these
anywhere.

This adds documentation for system vars to func_env
and for channel vars to func_channel so that they
appear along with the corresponding fields that would
be accessed using a function.

ASTERISK-29848 #close

Change-Id: I6997f925c4a45fffe71321861f5898a8b7182fa9