Update documentation to reflect new shared_info

The shared_info field was updated by crrev.com/c/3437855, but a piece of
documentation was missed.

Bug: 1251648
Change-Id: Ie83c922f998b5bb33fb95eeb9fe047548ac9b68f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3446081


Auto-Submit: Alex Turner <alexmt@chromium.org>
Commit-Queue: Alex Turner <alexmt@chromium.org>
Reviewed-by: Nan Lin <linnan@chromium.org>
Commit-Queue: Nan Lin <linnan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#968338}