[jsk_perception/doc/vqa_node] add explanation about model argument

[kochigami]

When I executed the below command following https://jsk-docs.readthedocs.io/projects/jsk_recognition/en/latest/jsk_perception/nodes/vqa_node.html#run-inference-container-on-another-host-or-another-terminal

kochigami@kochigami-ThinkPad-P16-Gen-1:~/catkin_ws/src/jsk_recognition/jsk_perception/docker$ ./run_jsk_vil_api --port 8888 --ofa_task caption --ofa_model_scale huge

I got the following error:

usage: run_jsk_vil_api [-h] [-p PORT] [--ofa_task OFA_TASK] [--ofa_model_scale {base,large,huge}] {ofa,clip}
run_jsk_vil_api: error: the following arguments are required: model

It seems that we need to set a model argument, but it seems that doc forgets it.
Thus, I added an explanation about model argument.
I'm not sure whether it is a good explanation, so please check this.

Related issue: #2849