The stated goal for pkgbits is to implement encoding / decoding of
primitives. However, pkgbits has knowledge of high-level details like
elements, sections, and file layout.

This change starts to clarify pkgbits by paring back documentation to
only those concepts which pkgbits owns. Further CLs are needed to shift
away logic that pkgbits should not own.

Change-Id: Id93003d080f58ffbd6327e2db1a4878500511619
Reviewed-on: https://go-review.googlesource.com/c/go/+/670176


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Mark Freeman <mark@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Mark Freeman <mark@golang.org>