kernel: document that busy-wait clock may differ from timeout clock

Some tests assumed that a delay with k_busy_wait() could be precisely
timed by a tick or cycle count, and vice versa.  This is not true for
all clock configurations on all Zephyr targets, so highlight the
potential variation.

As an example: on some platforms busy-wait is performed by a loop with
known duration in cycles of a known CPU frequency.  The CPU clock may
be unrelated to the clock that drives the system timer.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>