docs/ipc: Explicitly state that reply types correspond to message types
docs/ipc: Explicitly state that reply types correspond to message types

Help avoid some squinting to make sure everything actually matches.

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