Added copyright note for reference impl and brief documentation

ql/math/randomnumbers/xoshiro256starstaruniformrng.hpp

@@ -17,18 +17,47 @@
  FOR A PARTICULAR PURPOSE.  See the license for more details.
 */
 
+// NOTE: The following copyright notice applies to the
+// original C implementation https://prng.di.unimi.it/xoshiro256starstar.c
+// that has been used for this class.
+
+/*  Written in 2018 by David Blackman and Sebastiano Vigna ([email protected])
+
+To the extent possible under law, the author has dedicated all copyright
+and related and neighboring rights to this software to the public domain
+worldwide. This software is distributed without any warranty.
+
+See <http://creativecommons.org/publicdomain/zero/1.0/>. */
+
+/*! \file xoshiro256starstaruniformrng.hpp
+    \brief xoshiro256** uniform random number generator
+*/
+
 #ifndef quantlib_xoshiro256starstar_uniform_rng_hpp
 #define quantlib_xoshiro256starstar_uniform_rng_hpp
 
 #include <ql/methods/montecarlo/sample.hpp>
 #include <ql/types.hpp>
 
 namespace QuantLib {
+
+    //! Uniform random number generator
+    /*! xoshiro256** random number generator of period 2**256-1
+     *
+     * For more details see
+     *     https://prng.di.unimi.it/
+     * and its reference implementation
+     *     https://prng.di.unimi.it/xoshiro256starstar.c
+     *
+     * \test the correctness of the returned values is tested by checking them
+     * against the reference implementation in c.
+     * */
+
     class Xoshiro256StarStarUniformRng {
       public:
         typedef Sample<Real> sample_type;
-        /*! if the given seed is 0, a random seed will be chosen
-            based on clock() */
+
+        /*! if the given seed is 0, a random seed will be chosen based on clock() */
         explicit Xoshiro256StarStarUniformRng(unsigned long long seed = 0);
 
         Xoshiro256StarStarUniformRng(unsigned long long s0,
@@ -41,7 +70,7 @@ namespace QuantLib {
         sample_type next() const { return {nextReal(), 1.0}; }
 
         //! return a random number in the (0.0, 1.0)-interval
-        Real nextReal() const { return (Real(nextInt64() >> 11) + 0.5) * (1.0 / (1ULL << 53)); }
+        Real nextReal() const { return (Real(nextInt64() >> 11) + 0.5) * (1.0 / Real(1ULL << 53)); }
 
         //! return a random integer in the [0,0xffffffffffffffffULL]-interval
         unsigned long long nextInt64() const {