[DWRF] Better comment for the phantom issue

Summary:
@public
Adding more details about the phantom problem as per suggestion by Kevin Wilfing in prev. patch

Test Plan: None. This is simply adding a comment.

Reviewers: dilipj, kevinwilfong

Reviewed By: dilipj

Subscribers: dilipj, hive-devel@

Differential Revision: https://phabricator.fb.com/D1564629

Tasks: 4690285