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>

doc.go

diff --git a/doc.go b/doc.go
index 1ad48f5..eef99d9 100644
--- a/doc.go
+++ b/doc.go
@@ -4,22 +4,19 @@
 
 // Package xerrors implements functions to manipulate errors.
 //
-// This package supports transitioning to the Go 2 proposal for error values:
+// This package is based on the Go 2 proposal for error values:
 //   https://golang.org/design/29934-error-values
 //
-// Most of the functions and types in this package will be incorporated into the
-// standard library's errors package in Go 1.13; the behavior of this package's
-// Errorf function will be incorporated into the standard library's fmt.Errorf.
-// Use this package to get equivalent behavior in all supported Go versions. For
-// example, create errors using
+// These functions were incorporated into the standard library's errors package
+// in Go 1.13:
+// - Is
+// - As
+// - Unwrap
 //
-//    xerrors.New("write failed")
+// Also, Errorf's %w verb was incorporated into fmt.Errorf.
 //
-// or
+// Use this package to get equivalent behavior in all supported Go versions.
 //
-//    xerrors.Errorf("while reading: %v", err)
-//
-// If you want your error type to participate in the new formatting
-// implementation for %v and %+v, provide it with a Format method that calls
-// xerrors.FormatError, as shown in the example for FormatError.
+// No other features of this package were included in Go 1.13, and at present
+// there are no plans to include any of them.
 package xerrors // import "golang.org/x/xerrors"