Updates the `snapshots/` description now that some Dart tools are included as AOT snapshots. Also updates the `utils/` description to be more generic since wasm-opt is bundled as well.

TEST=ci

Change-Id: If2611e9ae139ede772061ee62644e91d9b051555
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/332180


Reviewed-by: Michael Thomsen <mit@google.com>
Auto-Submit: Parker Lougheed <parlough@gmail.com>
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Michael Thomsen <mit@google.com>