testing/quick: clarify that Config.MaxCount is from a flag

Document that the default quickcheck configuration is to run 100 times
and that there is a flag that configures it called "quickchecks".

Change-Id: I46fdab9d572e132ccc23ef907f9cc6b2d06b37c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/172698Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/testing/quick/quick.go

@@ -180,7 +180,8 @@ type Config struct {
 	MaxCount int
 	// MaxCountScale is a non-negative scale factor applied to the
 	// default maximum.
-	// If zero, the default is unchanged.
+	// If zero, the default is configured by the -quickchecks flag
+	// which defaults to 100.
 	MaxCountScale float64
 	// Rand specifies a source of random numbers.
 	// If nil, a default pseudo-random source will be used.