Fix PhabricatorStory::attachProjects undefined property issue

Summary:
This fixes the following issue:
Undefined property: Nasqueron\Notifications\Phabricator\PhabricatorStory::$instance

A code comment update was also needed for loadFromArray.

This is a follow-up for 67730e913d04.

Test Plan: Tested when fixing the notifications:payloads command for Phabricator

Reviewers: dereckson

Differential Revision: https://devcentral.nasqueron.org/D634