doc.go: update to current state Since Go 1.13 is out, we can state precisely what is in it. Fixes #34349. Change-Id: I71718fad98298d9b4c2300836c9fa2a01386b51b Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/199317 Reviewed-by: Damien Neil <dneil@google.com> Run-TryBot: Jonathan Amsterdam <jba@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org>