Documentation: Add comments to describe and recommend BlobReader/FileReader

Bug: 896479
Change-Id: Idc364a4fc9340ad19d121973492a60b9e9e3f905
Reviewed-on: https://chromium-review.googlesource.com/c/1369046
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617965}
3 files changed