Manager - Improve documentation for manager commands Getvar and Setvar.

The documentation for these commands did not make it clear that they could
accept expressions and functions. Modified to make this clear, but tried
not to be overly explicit.

ASTERISK-21178 #close
Reported by: Rusty Newton
Tested by: Rusty Newton

Review: https://reviewboard.asterisk.org/r/3854


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@419942 f38db490-d61c-443f-a65b-d21fe96a405b