Add comment explaining when custom allocator will be used.

Bug: b/321805728
Change-Id: I14bac51d496d78f07fef6fb7d3bb4f585a600627
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5463965


Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Mark Foltz <mfoltz@chromium.org>
Commit-Queue: Thorsten Sideboard <sideboard@google.com>
Cr-Commit-Position: refs/heads/main@{#1289441}