[Mac/iOS/GN] Document the new rules for create_bundle and runtime_deps.

BUG=611414
R=dpranke@chromium.org

Review-Url: https://codereview.chromium.org/2023003006
Cr-Original-Commit-Position: refs/heads/master@{#397258}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 57979207f98a847a40a8f5b57633cf7427c68c28