device: improve (z_)device_is_ready docs

Improve the documentation so that both z_device_is_ready and
device_is_ready are consistent with each other. Information about PM has
been removed since device_is_ready does not take into account the PM
status of a device.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
1 file changed