doc: reference: float: document ARM thread tag recommendation
doc: reference: float: document ARM thread tag recommendation

This commit adds a short note in reference/kernel/other/float.
The note instructs to apply pre-tagging of ARM threads to
indicate that they indend to use the FP services.

Signed-off-by: Ioannis Glaropoulos <5921cc8bab7e1d4329f52fd8f6268f9692e3de80@nordicsemi.no>

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