#7076: mention SystemRandom class near start of the module docs; reword change…

#7076: mention SystemRandom class near start of the module docs; reword change description for clarity.  Noted by Shawn Ligocki.

Doc/library/random.rst

@@ -52,7 +52,11 @@ known to fail some stringent randomness tests.  See the references below for a
 recent variant that repairs these flaws.
 
 .. versionchanged:: 2.3
-   Substituted MersenneTwister for Wichmann-Hill.
+   MersenneTwister replaced Wichmann-Hill as the default generator.
+
+The :mod:`random` module also provides the :class:`SystemRandom` class which
+uses the system function :func:`os.urandom` to generate random numbers
+from sources provided by the operating system.
 
 Bookkeeping functions: