Add doc aliases for DefaultRandomSource, for people looking for randomness

`getrandom` is both a crate for this and the name of the underlying
syscall on various OSes. `getentropy` and `arc4random` are common
library calls on various OSes.

Author: joshtriplett

library/std/src/random.rs

@@ -56,6 +56,7 @@ use crate::sys::random as sys;
 ///
 /// [`getrandom`]: https://www.man7.org/linux/man-pages/man2/getrandom.2.html
 /// [`/dev/urandom`]: https://www.man7.org/linux/man-pages/man4/random.4.html
+#[doc(alias = "getrandom", alias = "getentropy", alias = "arc4random")]
 #[derive(Default, Debug, Clone, Copy)]
 #[unstable(feature = "random", issue = "130703")]
 pub struct DefaultRandomSource;