Add the Doxygen note to non-supported RTOS kernel APIs.
Add the Doxygen note to non-supported RTOS kernel APIs.

mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,kjbracey-arm/mbed,kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed