docs/ipc: Add descriptions of all message payloads
docs/ipc: Add descriptions of all message payloads

These seemed to be inconsistently specified.

Particularly, the "SYNC" message payload was not described anywhere.

Even when the payload is empty, it is helpful to specify that
explicitly, as it prevents the reader from having to guess whether if
it's really empty, or otherwise somehow implicitly obvious.

The "Reply format" section is now "Messages and replies", and covers
both the reply format and the format of sent messages.

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