math/rand: rewrite the math/rand package comment to say what it's good for

It currently says only what it wasn't good for, which is not helpful.

Change-Id: I468c7f385c14eaca99788a94d53c30b729ed0944
Reviewed-on: https://go-review.googlesource.com/c/go/+/466276
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>

Author: robpike

src/math/rand/rand.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package rand implements pseudo-random number generators unsuitable for
-// security-sensitive work.
+// Package rand implements pseudo-random number generators suitable for tasks
+// such as simulation, but it should not be used for security-sensitive work.
 //
 // Random numbers are generated by a [Source], usually wrapped in a [Rand].
 // Both types should be used by a single goroutine at a time: sharing among