samples: boards: stm32: serial_wakeup: Fix nucloe_wb55rg configuration

On STM32WB55 series, wakeup in stop mode is not supported.
Disable this state in order to support this sample.

Add comments to other sections of the configuration.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>
1 file changed