[rotang] Documentation and bugfix of the modifier UI.

Turns out I was looking at the wrong element when confirming my last
change.. So when can we have TypeScript? :)

https://rota-ng-staging.googleplex.com <- Live here.

Bug: 920632
Change-Id: Ib613f2965d745d7a06086eac183fd4fa352ab337
Reviewed-on: https://chromium-review.googlesource.com/c/1441656
Reviewed-by: Tiffany Zhang <zhangtiff@chromium.org>
Commit-Queue: Ola Karlsson <olakar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#20387}