I was confused by the previous state of the docs since I imagined that
there is still a race possible. Thus, the images are updated to
explicitly include the IPC reply from i3 (since that is an important
synchronisation component). I also tried to clarify some of the text.
Fixes: https://github.com/i3/i3/issues/4365
Signed-off-by: Uli Schlachter <psychon@znc.in>