ctf: Add comments to CTFClock

ctf: Add comments to CTFClock

I was asked, is it deliberate that CTFClock's constructor is empty and
what is a clock, how does it work. This patch attempts to answer some of
the frequently asked questions.

Change-Id: I42dd8254f73df2d92d5e2497600b0b6ffc3682c4
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/70369
Reviewed-by: Hudson CI
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>