lp ticker - fix doc for sleep and set interrupt #78
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I had various questions about this 2 functions, which led me to check targets implementations and found various things (more here ARMmbed/minar-platform-mbed#17).
It was not clear what is the intention with now and the time. Because
there are various timers (counter running up with matching interrupt, or
a timer with deltas (from a value count down to 0)). This should
clarify what is expected to do, our tests can test this.
@autopulated @bogdanm @marcuschangarm