doc: clarify experimental API elements in vm.md

Remove redundant "should not be considered stable" (it immediately
follows a clause saying that it is "experimental") and replace with a
more clear (to me, at least) statement that we don't recommend using the
feature in production.

PR-URL: https://github.com/nodejs/node/pull/35594


Reviewed-By: James M Snell <jasnell@gmail.com>