As a follow-up to https://crrev.com/c/3625835, document how we
internally encode Wasm opcodes in the WasmOpcode enum. In particular,
it's important for the mapping to be bijective.

R=thibaudm@chromium.org
CC=gdeepti@chromium.org

Bug: v8:12284
Change-Id: Ic4bcd70211e83b1eabb45204bdcce3209a4432b3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3647360Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80635}