cmd/internal/objabi/doc.go has comments decribing the (old)
object file format. But cmd/internal/objabi has nothing to do
with object files, and never did. Delete.
Move some comment to cmd/internal/goobj, where the (new) object
file format is actually defined, and update to reflect the
current status.
Change-Id: Ied96089df4be35e5d259a572ed60ee00f2cd0d1d
Reviewed-on: https://go-review.googlesource.com/c/go/+/249958 Reviewed-by: Than McIntosh <thanm@google.com>