autoValue documentation for docId is misleading.

[OliverColeman]

Just spent some time figuring out why a findOne(this.docId) in an autoValue function was causing an error. The documentation for this.docId says docId: The _id property of the document being inserted or updated but it turns out it's not just the _id of the document, it's a filter query like { '$in': [ 'rg8gSYhGaNQaJ3X8p' ] }.

[coagmano]

I think the docs represent the original intention, but if an update uses a query in the selector, it will be used instead as a result of this code:

  } else if (type === "update" && selector) {
    docId = typeof selector === 'string' || selector instanceof Mongo.ObjectID ? selector : selector._id;
  }

source

Not sure what the "correct" behaviour here should look like

Want to make a PR to update the docs?

[CaptainN]

Is this the correct behavior? I would think the property should be made to always return a document id, as documented, and intuitively.