[Scenario] Extend docstring for Scenario.add_output
[Scenario] Extend docstring for Scenario.add_output

Let's make it more documented.

Change-Id: I2e92e4bd88b67136766790889aaf72f6130fb77a

eayunstack/rally,eonpatapon/rally,openstack/rally,gluke77/rally,eayunstack/rally,eonpatapon/rally,yeming233/rally,vganapath/rally,openstack/rally,vganapath/rally,gluke77/rally,gluke77/rally,vganapath/rally,openstack/rally,yeming233/rally,gluke77/rally,vganapath/rally,eayunstack/rally,openstack/rally