Add example for -ra usage to the docs

doc/en/usage.rst

@@ -152,6 +152,42 @@ making it easy in large test suites to get a clear picture of all failures, skip
 
 Example:
 
+.. code-block:: python
+
+    # content of test_example.py
+    import pytest
+
+
+    @pytest.fixture
+    def error_fixture():
+        assert 0
+
+
+    def test_ok():
+        print("ok")
+
+
+    def test_fail():
+        assert 0
+
+
+    def test_error(error_fixture):
+        pass
+
+
+    def test_skip():
+        pytest.skip("skipping this test")
+
+
+    def test_xfail():
+        pytest.xfail("xfailing this test")
+
+
+    @pytest.mark.xfail(reason="always xfail")
+    def test_xpass():
+        pass
+
+
 .. code-block:: pytest
 
     $ pytest -ra