anchor [nfc]: Correct the jsdoc on FIRST_UNREAD_ANCHOR.
anchor [nfc]: Correct the jsdoc on FIRST_UNREAD_ANCHOR.

This jsdoc didn't match the name at all!  It turns out the name was
right... and the jsdoc described what, confusingly, the very same
value means in the actual Zulip API.

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