Add code comments to make sure Rust <> JS "reserved usernames" are mirrored

Summary:
adding comments to help ensure the two lists stay in sync

Note that I could not add comments to the actual reserved_usernames.json file
since comments aren't allowed in JSON notation

Depends on D8481

Test Plan: N/A, just comments

Reviewers: ashoat, jon

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8482