time: add more docs on Sleep
time: add more docs on Sleep

Merge the comment from runtime/time.goc ("at least")
and also note that negative is okay and won't crash.
I see people going out of their way to avoid passing
a negative value to Sleep.

R=golang-dev, adg, r, alex.brainman
CC=golang-dev
https://codereview.appspot.com/13271045

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