Document:
1. The new debug and logfile options with their usages
2. New json format and its usage and
3. update "GlusterFS, Device URL Syntax" section in "Invocation"

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>