Missing documentation of mutateIn upsert/insert options.

https://forums.couchbase.com/t/subdoc-on-non-existing-document/19539 

A user is trying to mutate fields in a doc that may/may not exist yet. This is the basic scenario:

1. mutate fields (if doc exists then everything ok)
2. if mutate failed then insert new doc with those fields

We should document this as it seems a useful scenario for users.

In the forum we can see that this has been suggested by Brett:

bucket.mutateIn('mydoc', {upsert: true})

  .upsert('foo.bar', 10, {createParents: true})

  .upsert('foo.baz', 20, {createParents: true})

  .execute((err) => {

    // Completed!

  });

Which seems to be what the user was looking for (also see the linked issue, seems API docs needed to be updated).