crypto: document rest of rand submodule (#8580)

vlib/crypto/rand/utils.v

@@ -7,6 +7,7 @@ module rand
 import math.bits
 import encoding.binary
 
+// int_u64 returns a random unsigned 64-bit integer `u64` read from a real OS source of entropy.
 pub fn int_u64(max u64) ?u64 {
 	bitlen := bits.len_64(max)
 	if bitlen == 0 {