Document current-params option for trigger-build

There was some documentation missing, so I had to find this option when I was
trying to add it myself, in the source code. Nasty!

Added documentation so nobody else has to do that.

Change-Id: I9b6a0227479b600fed213d926f5cd22981db612e
Reviewed-on: https://review.openstack.org/23963
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: Antoine Musso <hashar@free.fr>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins