Add commments to ReturnToAppPanel per feedback in earlier CLs

This was intended to be part of https://crrev.com/1318417, but due to a
mistake when rebasing CLs locally, accidentally became merged into
another unrelated commit.

Change-Id: I1d76eeb6384d8d9622159e89070e4e21216709f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5660885
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jiaming Cheng <jiamingc@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1320657}