lttng: bug 495211 document the critical flow view

lttng: bug 495211 document the critical flow view

The documentation introduces the view and the required steps to actually load
and run the wait analysis on a process.

Change-Id: I860ef950ef48381c5b5da3009d0a780970684f94
Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/74568
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>