doc/go1.13: mention the effect of testing.Init on package initialization

Fixes #31859

Change-Id: I228ed93f9466d1a22f8796964f5d2f056ab9b62f
Reviewed-on: https://go-review.googlesource.com/c/go/+/186818


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Caleb Spare <cespare@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>