Clarify that it's not necessary to subclass from TestCase to create a

test case. As Jeremy put it: "subclassing is an implementation
technique, not a type declaration".

Doc/lib/libunittest.tex

@@ -35,7 +35,8 @@ starting a server process.
 A \dfn{test case} is the smallest unit of testing.  It checks for a
 specific response to a particular set of inputs.  PyUnit provides a
 base class, \class{TestCase}, which may be used to create new test
-cases.
+cases. You may provide your own implementation that does not subclass
+from \class{TestCase}, of course.
 
 \term{test suite}
 A \dfn{test suite} is a collection of test cases, test suites, or