math/rand: add a comment for the i=0 iteration

Fixes #13215

Change-Id: I126117d42e7c1e69cbc7fad0760e225b03ed15bd
Reviewed-on: https://go-review.googlesource.com/16852Reviewed-by: Keith Randall <khr@golang.org>

src/math/rand/rand.go

@@ -148,6 +148,11 @@ func (r *Rand) Float32() float32 {
 // Perm returns, as a slice of n ints, a pseudo-random permutation of the integers [0,n).
 func (r *Rand) Perm(n int) []int {
 	m := make([]int, n)
+	// In the following loop, the iteration when i=0 always swaps m[0] with m[0].
+	// A change to remove this useless iteration is to assign 1 to i in the init
+	// statement. But Perm also effects r. Making this change will affect
+	// the final state of r. So this change can't be made for compatibility
+	// reasons for Go 1.
 	for i := 0; i < n; i++ {
 		j := r.Intn(i + 1)
 		m[i] = m[j]