Improve documentation for non_negative_derivative and non_negative_difference

__Actual behavior:__
Documentation is not clear what happens when the difference/derivative is negative when using non_negative_derivative/non_negative_difference functions.

The documentation states "NON_NEGATIVE_DERIVATIVE() returns only positive rates of change or rates of change that equal zero.", but it is unclear what is returned when the derivative/difference is negative (is null or zero returned?).

__Expected behavior:__
Documentation should be more clear. The comment at https://github.com/influxdata/influxdb/issues/7393#issuecomment-251440102 suggests that the functions return null, but this needs to be confirmed and added to the documentation.