Improve the docs for I3_SYNC

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>

docs/testsuite

@@ -481,7 +481,7 @@ an i3 crash resulting in the testcase being unable to communicate with i3 via
 IPC anymore.
 
 [[i3_sync]]
-== Appendix A: The i3 sync protocol
+== Appendix A: The I3_SYNC protocol
 
 Consider the following situation: You open two windows in your testcase, then
 you use +focus left+ and want to verify that the X11 focus has been updated
@@ -499,9 +499,9 @@ is($x->input_focus, $left->id, 'left window focused');
 However, the test fails. Sometimes. Apparently, there is a race condition in
 your test. If you think about it, this is because you are using two different
 pieces of software: You tell i3 to update focus, i3 confirms that, and then you
-ask X11 to give you the current focus. There is a certain time i3 needs to
+ask X11 to give you the current focus. There is a certain time that the X11
-update the X11 state. If the testcase gets CPU time before X11 processed i3's
+server needs to process the requests from i3. If the testcase's request for the
-requests, the test will fail.
+input focus is processed before i3's requests, the test will fail.
 
 image::i3-sync.png["Diagram of the race condition", title="Diagram of the race condition"]
 
@@ -531,10 +531,10 @@ less robust.
 
 The real solution for this problem is a mechanism which I call "the i3 sync
 protocol". The idea is to send a request (which does not modify state) via X11
-to i3 which will then be answered. Due to the request's position in the event
+to i3 which will then be answered, again via X11. Because this answer is
-queue (*after* all previous events), you can be sure that by the time you
+generated via an X11 request, it will be sent to the X11 server *after* all
-receive the reply, all other events have been dealt with by i3 (and, more
+previous requests. Thus, you can be sure that by the time you receive the reply,
-importantly, X11).
+all other events have been dealt with by i3 (and, more importantly, X11).
 
 image::i3-sync-working.png["Diagram of the i3 sync solution", title="Diagram of the i3 sync solution"]