Document that 'never' is valid for expires_after in rare cases.

Bug: 923083
Change-Id: Ic774c0bda4ce2f0fb794923d31c92511fabc76cb
Reviewed-on: https://chromium-review.googlesource.com/c/1447845
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630047}
2 files changed