[file manager] Add a note about cached metadata usage

Bug: None
Change-Id: I5903720d40ffff97da803c44f9c5836718def27b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4563133


Auto-Submit: Oleg Davydov <burunduk@chromium.org>
Reviewed-by: Austin Tankiang <austinct@chromium.org>
Commit-Queue: Austin Tankiang <austinct@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148935}