doc: mention testing/quick RNG seeding change in Go 1.9 notes

Also reword the testing/quick.Config field docs to conform to the
normal subject-first style. Without that style, godoc links
/pkg/testing/quick/#Config.Rand to the wrong line, since it doesn't
recognize the preceding comment as necessarily being attached.

Fixes #20809

Change-Id: I9aebbf763eed9b1ab1a153fa11850d88a65571c6
Reviewed-on: https://go-review.googlesource.com/46910Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.9.html

@@ -792,6 +792,14 @@ CL 40331: https://golang.org/cl/40331: cmd/link,runtime/cgo: enable PT_TLS gener
       limited to the [-2<sup>62</sup>, 2<sup>62</sup>) range.
     </p>
 
+    <p>
+      In previous releases, using a nil
+      <a href="/pkg/testing/quick/#Config.Rand"><code>Config.Rand</code></a>
+      value caused the same deterministic random number generator to be used.
+      It now uses a random number generator seeded on the current time.
+      For the old behavior, use <code>rand.New(rand.NewSource(0))</code>.
+    </p>
+
 </dl><!-- testing/quick -->
 
 <dl id="text/template"><dt><a href="/pkg/text/template/">text/template</a></dt>

src/testing/quick/quick.go

@@ -175,19 +175,20 @@ func sizedValue(t reflect.Type, rand *rand.Rand, size int) (value reflect.Value,
 
 // A Config structure contains options for running a test.
 type Config struct {
-	// MaxCount sets the maximum number of iterations. If zero,
-	// MaxCountScale is used.
+	// MaxCount sets the maximum number of iterations.
+	// If zero, MaxCountScale is used.
 	MaxCount int
-	// MaxCountScale is a non-negative scale factor applied to the default
-	// maximum. If zero, the default is unchanged.
+	// MaxCountScale is a non-negative scale factor applied to the
+	// default maximum.
+	// If zero, the default is unchanged.
 	MaxCountScale float64
-	// If non-nil, rand is a source of random numbers. Otherwise a default
-	// pseudo-random source will be used.
+	// Rand specifies a source of random numbers.
+	// If nil, a default pseudo-random source will be used.
 	Rand *rand.Rand
-	// If non-nil, the Values function generates a slice of arbitrary
-	// reflect.Values that are congruent with the arguments to the function
-	// being tested. Otherwise, the top-level Value function is used
-	// to generate them.
+	// Values specifies a function to generate a slice of
+	// arbitrary reflect.Values that are congruent with the
+	// arguments to the function being tested.
+	// If nil, the top-level Value function is used to generate them.
 	Values func([]reflect.Value, *rand.Rand)
 }