HPCC-2566 Allow attribute values to be specified on the command line

Also allow ^ as a synonym for the root scope.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
It's the help text, not the manual... there to remind people what the function is not provide a complete description of how to use it.

(we should consider creating a manpage for eclcc at some point)