Update enginevariants.html.md

[luto65]

added description of the commandline and examples

[Wei-1]

Maybe this will make more sense?

In order to deploy multiple variants,
one must specify a JSON file containing different variants,
a port,
and an instance-id to an engine.
At the end of the training,
the instance-id will be stored in the `````id````` column
of the `````pio_meta_engineinstances````` table.