docs/firmware: Update FW_JUMP documentation

Add a tip for OpenSBI's FW_JUMP which helps
users avoid overwriting the kernel.

Signed-off-by: Nylon Chen <nylon.chen@sifive.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>