change sensor HAL documentation to allow setting polling rate for "non continuous...

change sensor HAL documentation to allow setting polling rate for "non continuous" sensors

the current API / Documentation would not allow to set the
rate of "non continuous" sensors.

Most sensors HAL implementation already support changing the rate
(technically they're not compliant with the doc), so it's easier
to change the doc (and also allows more control).

Change-Id: I4ad324ddc3b569ec61770b3cf927c06f8ff4af4e