doc,vm: warn more prominently against executing untrusted code

[bnoordhuis]

The vm module is not a security mechanism. There used to be a prominent warning against executing untrusted code but it seems to have been removed in recent years.

I think it would be good to bring it back because the language around sandboxing can mislead users into thinking the vm module is something it's not.

[krydos]

Sound good as first contribution. Can I try to do it please?

Will it be ok to put this as Note: ... somewhere at the end of the first section?

[bnoordhuis]

@krydos Yes, that's a good place for it.

[bnoordhuis]

Fixed by 2e74b0d.