[landing] Add a comment to siwe.css explaining where the class names come from

Summary: Responding to @atul's diff comment here: https://phab.comm.dev/D6010?id=20046#inline-40665

Test Plan: Opened file to confirm both class names came from there

Reviewers: atul

Reviewed By: atul

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6021