clocksource: document some basic timekeeping concepts
This adds some documentation about clock sources, clock events,
the weak sched_clock() function and delay timers that answers
questions that repeatedly arise on the mailing lists.
Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Nicolas Pitre <nico@fluxnic.net> Cc: Colin Cross <ccross@google.com> Cc: John Stultz <john.stultz@linaro.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ingo Molnar <mingo@redhat.com> Signed-off-by: Linus Walleij <linus.walleij@linaro.org> Acked-by: Nicolas Pitre <nico@linaro.org> Signed-off-by: John Stultz <john.stultz@linaro.org>