Improve comments on encryption-related SyncUserSettings methods

(and some related methods elsewhere)

Bug: 884159
Change-Id: I2b87b9fc9302f35e5fd3862035ef84418e5ffac8
Reviewed-on: https://chromium-review.googlesource.com/c/1432844
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625694}
3 files changed