doc/go1.16: document BuildID in 'go list -export'

doc/go1.16: document BuildID in 'go list -export'

This corresponds to the feature in https://golang.org/cl/263542, since
this can be a noteworthy change to people writing tools to inspect Go
builds.

Also amend the wording to clarify that build IDs are for an entire
compiled package, not just their export data or object file.

Change-Id: I2eb295492807d5d2997a35e5e2371914cb3ad3a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/264158
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>