doc: eliminate many "expected" doc gen warnings
doc: eliminate many "expected" doc gen warnings

Adding NET_MGMT_DEFINE_REQUEST_HANDLER to the doxygen configuration
list of pre-defined macros eliminates 800 lines of "expected" warning
messages from the document generation output.  Generated API docs
are unchanged. Cool.

Remember this if/when other macros pop up as a problem for the
doxygen/breathe/sphinx API generation tools.

Signed-off-by: David B. Kinder <c56c8b29908a5b2af1b63001268d73a0b7f972ba@intel.com>

ldts/zephyr,Vudentz/zephyr,ldts/zephyr,ldts/zephyr,galak/zephyr,aceofall/zephyr-iotos,kraj/zephyr,explora26/zephyr,explora26/zephyr,mbolivar/zephyr,fbsder/zephyr,finikorg/zephyr,mbolivar/zephyr,punitvara/zephyr,Vudentz/zephyr,finikorg/zephyr,aceofall/zephyr-iotos,zephyriot/zephyr,punitvara/zephyr,explora26/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,Vudentz/zephyr,fbsder/zephyr,nashif/zephyr,finikorg/zephyr,zephyriot/zephyr,zephyriot/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,fbsder/zephyr,punitvara/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,zephyriot/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,aceofall/zephyr-iotos,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,kraj/zephyr,kraj/zephyr,nashif/zephyr,nashif/zephyr,explora26/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,mbolivar/zephyr,aceofall/zephyr-iotos,Vudentz/zephyr,mbolivar/zephyr,ldts/zephyr,nashif/zephyr,zephyriot/zephyr,finikorg/zephyr,Vudentz/zephyr,fbsder/zephyr,fbsder/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr