vm: document vm2's changes.

- The caveats no longer apply.
- Document options arguments, including `displayErrors` and the
  different things it means in each place.
- Re-did examples to be more on point, e.g. `runInContext` example
  runs multiple scripts in the same context.
- Documented how `vm.createContext`s meaning has substantially changed,
  and is now more of a "contextifier" than a "creator."
- Reordered vm functions to be readable in order; the concept of
  contextifying needs to come before `runInContext` and
  `runInNewContext`.
- Documented new `vm.isContext`.
- Documented the `vm.Script` constructor, instead of `createScript`,
  since factory methods are silly and we wanted to document the class's
  methods anyway.
- Documented `script.runInContext`.
- Change stability to stable, if I may be so bold.