doc: bluetooth/mesh: Add mod_sub_ow_va document
doc: bluetooth/mesh: Add mod_sub_ow_va document

Add mod_sub_ow_va command documentation.

Signed-off-by: Lingao Meng <d107961e9e4fef34975d9e927886461599dc7eda@xiaomi.com>

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