`addPatch` with handler path is poorly defined

The [`addPatch` documentation](https://www.elastic.co/guide/en/apm/agent/nodejs/current/agent-api.html#apm-add-patch) says that the `handler` argument must be:
>handler - function or string Must be a patch function or a path to a module exporting a patch function

But exactly what "a path to a module exporting a patch function" means is ambiguous as it's unclear what a relative module path would be relative to.