clarify how to properly create random variable objects

doc/manual/source/random-variables.rst

@@ -105,6 +105,41 @@ use a single RNG and streams and substreams from it.
 
 .. _seeding-and-independent-replications:
 
+Creating random variables
+*************************
+
+|ns3| supports a number of random variable objects from the base class
+:cpp:class:`RandomVariableStream`.  These objects derive from 
+:cpp:class:`ns3::Object` and are handled by smart pointers.
+
+The correct way to create these objects is to use the templated 
+`CreateObject<>` method, such as:
+
+::
+
+  Ptr<UniformRandomVariable> x = CreateObject<UniformRandomVariable> ();
+
+then you can access values by calling methods on the object such as:
+
+::
+
+  myRandomNo = x->GetInteger ();
+  
+
+If you try to instead do something like this:
+
+::
+
+  myRandomNo = UniformRandomVariable().GetInteger ();
+
+your program will encounter a segmentation fault, because the implementation
+relies on some attribute construction that occurs only when `CreateObject`
+is called.
+
+Much of the rest of this chapter now discusses the properties of the
+stream of pseudo-random numbers generated from such objects, and how to
+control the seeding of such objects.
+
 Seeding and independent replications
 ************************************