device: improve (z_)device_is_ready docs
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 <bedb45f43a286e49f4dcda039a436b216ae61ef3@nordicsemi.no>

galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr