doc/application: add a note about choosing emulator platform
doc/application: add a note about choosing emulator platform

New targets have been added that allow the user to choose a specific
emulator platform. Inform about this in the docs.

Signed-off-by: Filip Kokosinski <15ac8876ac1a5305e50b11a5381cbe599b2262b1@antmicro.com>

finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr