unread [nfc]: Write jsdoc on the various selectors.
unread [nfc]: Write jsdoc on the various selectors.

Also several TODO comments, where it looks like the actual semantics
and intended semantics differ.

One of those looks to be the cause of a bug we've gotten reports of!
Namely #3589.  So, hooray for closely reading code.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile