Improve DockerHubListener comments

Summary:
Class comments were based on the Phabricator listener ones,
with some leftovers.

This is a follow-up for e4b94f5c52b9.

Ref T959.

Test Plan: no op

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T959

Differential Revision: https://devcentral.nasqueron.org/D1785