Document single-method usage

It is possible to run a single test method in a class, but the syntax is
not glaringly obvious and documentation would help people out.

doc/en/usage.rst

@@ -52,6 +52,8 @@ Several test run options::
     py.test test_mod.py::test_func  # only run tests that match the "node ID",
                                     # e.g "test_mod.py::test_func" will select
                                     # only test_func in test_mod.py
+    py.test test_mod.py::TestClass::test_method  # run a single method in
+                                                 # a single class
 
 Import 'pkg' and use its filesystem location to find and run tests::