[md-settings] Clarify Password Saving and Autofill Toggles

This change clarifies the wording around the password saving and
autofill toggles.

Bug: 822465
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I91b31fe61cd0754239f7908e8c04c7e69b72f670
Reviewed-on: https://chromium-review.googlesource.com/970541
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Vaclav Brozek <vabr@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544661}