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>
1 file changed