doc/go1.13.html: describe error-value additions

A brief description at the top the Standard Library section of the
changes to support error wrapping.

Fixes #33365.

Change-Id: Id5a3b2fe148d9bfb949f2cfc9e5d8a1613a0e219
Reviewed-on: https://go-review.googlesource.com/c/go/+/188798


Reviewed-by: Ian Lance Taylor <iant@golang.org>