debug/gocore: document that a gocore.Object is reachable

Unreachable objects don't get an Object.

Change-Id: Ia9c122d9619d186c02345cb9ed5a857ac599b6b8
Reviewed-on: https://go-review.googlesource.com/98955
Reviewed-by: Peter Weinberger <pjw@google.com>
1 file changed