math/rand/v2: document PCG and ChaCha8 tradeoffs

Document that ChaCha8 is the general-purpose source resistant to
prediction, and that PCG is faster but unfit for security-relevant
purposes.

Fixes #79891

Change-Id: Ic4133dd9bd860c2643c622f799f2c54b1678e4f0
GitHub-Last-Rev: 93d225108aaef7862f4f148cd926f420e58af831
GitHub-Pull-Request: golang/go#79926
Reviewed-on: https://go-review.googlesource.com/c/go/+/789040
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>

src/math/rand/v2/rand.go

diff --git a/src/math/rand/v2/rand.go b/src/math/rand/v2/rand.go
index 662e84f..991c184 100644
--- a/src/math/rand/v2/rand.go
+++ b/src/math/rand/v2/rand.go
@@ -12,6 +12,9 @@
 // Top-level functions, such as [Float64] and [Int],
 // are safe for concurrent use by multiple goroutines.
 //
+// The [ChaCha8] source is a general-purpose source resistant to prediction.
+// The [PCG] source is faster but unfit for security-relevant purposes.
+//
 // This package's outputs might be easily predictable regardless of how it's
 // seeded. For random numbers suitable for security-sensitive work, see the
 // [crypto/rand] package.