hwmon: (pmbus) Add documentation for new flags

Add documentation for the new pmbus flags PMBUS_WRITE_PROTECTED,
PMBUS_NO_CAPABILITY and PMBUS_READ_STATUS_AFTER_FAILED_CHECK

Signed-off-by: Erik Rosen <erik.rosen@metormote.com>
Link: https://lore.kernel.org/r/20210507194023.61138-3-erik.rosen@metormote.com


[groeck: Added newline at end of file]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>