This addresses comments from [0] by extending comments to also
describe embedded builtins in code.h, and by improving language
around various meaning of 'metadata':

- The Code object's metadata section is still called 'metadata'.
- The embedded blob's table of layout descriptions for builtins is
  now called 'layout descriptions'.
- The embedded blob's data section (containing hashes and layout
  descriptions) is now called 'data' section.

[0] chromium-review.googlesource.com/c/v8/v8/+/2491025

Bug: v8:11036
Change-Id: Ibe84fddb9784cc5d3b66482612dcdb7a2e8d14ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2501284
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70793}