std: clarify the strength of DefaultRandomSource

library/std/src/random.rs

@@ -10,12 +10,14 @@ use crate::sys::random as sys;
 
 /// The default random source.
 ///
-/// This asks the system for the best random data it can provide, meaning the
-/// resulting bytes *should* be usable for cryptographic purposes. Check your
-/// platform's documentation for the specific guarantees it provides. The high
-/// quality of randomness provided by this source means it is quite slow. If
-/// you need a larger quantity of random numbers, consider using another random
-/// number generator (potentially seeded from this one).
+/// This asks the system for random data suitable for cryptographic purposes
+/// such as key generation. If security is a concern, consult the platform
+/// documentation below for the specific guarantees your target provides.
+///
+/// The high quality of randomness provided by this source means it can be quite
+/// slow. If you need a large quantity of random numbers and security is not a
+/// concern,  consider using an alternative random number generator (potentially
+/// seeded from this one).
 ///
 /// # Underlying sources
 ///