doc: Bluetooth: Mesh: Align wording for models instantiation req

According to https://github.com/zephyrproject-rtos/zephyr/pull/61886#issuecomment-1713302331


we need to use "must only" for models that spec states:
`If supported, ... shall be supported by the primary element and shall
not be supported by any secondary element`.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>