testing: update documentation to be clear about when parallel subtests run

What version of Go are you using (go version)?

go version go1.9.2 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/zwass/dev/go"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.9.2/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.9.2/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/4k/2q3ctn7d5_xb03_zsb3v6lgr0000gn/T/go-build252941481=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

go test -v with the following code:

package main

import (
	"fmt"
	"sync"
	"testing"
)

func TestHang(t *testing.T) {
	t.Parallel()
	var wg sync.WaitGroup
	wg.Add(1)
	fmt.Println("added wg")
	t.Run("", func(t *testing.T) {
		fmt.Println("deferred done")
		t.Parallel()
		fmt.Println("set parallel")
		wg.Done()
	})

	fmt.Println("waiting")
	wg.Wait()
	fmt.Println("waiting complete")
}

What did you expect to see?

All statements print and test completes

What did you see instead?

Test hangs after "deferred done" and "waiting" are printed.

This is explained by the following comment in the blog post introducing subtests.

A parallel test never runs concurrently with a sequential test and its execution is suspended until its calling test function, that of the parent test, has returned.

But I cannot find this mentioned anywhere in the documentation for the testing package.

It seems like we at least need to update those docs with the appropriate details.