arch: posix: posix_cheats: Add some comments
arch: posix: posix_cheats: Add some comments

Add a few comments in posix_cheats.h explaining why the file
exists and how it is used.

Signed-off-by: Alberto Escolar Piedras <52be3174920ed06cbdfbcf9622a1a6c68e942617@oticon.com>

GiulianoFranchetto/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,explora26/zephyr,ldts/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,finikorg/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,explora26/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,explora26/zephyr