docs: driver-api: generic-counter: Update Count and Signal data types

Count data is now always represented as an unsigned integer, while
Signal data is either SIGNAL_LOW or SIGNAL_HIGH. In addition,
clarification changes and additions are made to better explain the
theory of the Generic Counter interface and its use.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>