doc: doxygen: Hide implementation detail symbols
doc: doxygen: Hide implementation detail symbols

Hide all symbols whose names begin with an underscore.  Although these
might be documented, they shouldn't be used by applications, as there's
no guarantee they will be available in subsequent versions, or maintain
API and ABI compatibility.

Signed-off-by: Leandro Pereira <51c7437a02b34df685e6b4ecb2e9bca23efe2e1a@intel.com>

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