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 <[email protected]>
Cc: Nicolas Pitre <[email protected]>
Cc: Colin Cross <[email protected]>
Cc: John Stultz <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
Signed-off-by: Linus Walleij <[email protected]>
Acked-by: Nicolas Pitre <[email protected]>
Signed-off-by: John Stultz <[email protected]>

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