Update validators

[HunSpeedi]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

In the validation docs you mention that:

However, when running update validators, the document being updated may not be in the server's memory, so by default the value of this is not defined.

If I understand correctly now Mongoose always uses context = 'query'.
So wouldn't be the following statement more accurate:

However, when running update validators, this refers to the query object instead of the document.

Also then an additional example could be added:

 /**
   * Since Mongoose always uses `context = 'query'` when running update validators `this` refers to the query object!
   */

   it('Update validation with query object', function(done) {
    // acquit:ignore:start
    const toySchema = new Schema({
      color: String,
      name: String
    });
    // acquit:ignore:end
    toySchema.path('color').validate(function(value) {
      // When running update validators `this` refers to the query object.
      if (this.getUpdate().$set.name.toLowerCase().indexOf('red') !== -1) {
        return value === 'red';
      }
      return true;
    });

    const Toy = db.model('ActionFigure', toySchema);

    const update = { color: 'blue', name: 'Red Power Ranger' };
    const opts = { runValidators: true };

    Toy.updateOne({}, update, opts, function(error) {
      assert.ok(error.errors['color']);
      // acquit:ignore:start
      done();
      // acquit:ignore:end
    });
  });

If my conclusion is right I can make a PR with the mentioned changes.
Thanks in advance!!