[sdk] comments on build definitions

No logic changes. Simply adding comments to help describe existing items.

Change-Id: I8f921832f81eb1d4bd6cceb3fcf64a4892be0286
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/524320
Reviewed-by: Misha Gridnev <gridman@google.com>
Reviewed-by: Adam Barth <abarth@google.com>
Commit-Queue: Dave Schuyler <dschuyler@google.com>