docs/userguide: Document that i3 can accept RandR output names
docs/userguide: Document that i3 can accept RandR output names

Airblader/i3,Airblader/i3-original,stapelberg/i3,Airblader/i3,Airblader/i3,cornerman/i3,cornerman/i3,cornerman/i3,Airblader/i3-original,cornerman/i3,i3/i3,stapelberg/i3,Airblader/i3,stapelberg/i3,i3/i3,Airblader/i3-original,i3/i3,Airblader/i3-original,i3/i3,stapelberg/i3,Airblader/i3-original