cmd/go: add documentation for Embed fields in go list output

This change the struct fields for EmbedPatterns and EmbedFiles
to the Package struct listed in the go list documentation that
specifies the fields available to the go list template.

Fixes #43081

Change-Id: I89c325a9d6292a6ce484ee588b172d2f84e2333a
Reviewed-on: https://go-review.googlesource.com/c/go/+/282195


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>