Correct help for --runxfail flag

The help for the '--runxfail' flag is incorrect. The default behaviour is to
run tests marked as 'xfail' but to ignore the results. This flag alters that
behaviour in two ways:

1. *Only* tests marked 'xfail' are run.
2. The results of these tests are reported as if they weren't marked 'xfail'.

This commit updates the help to describe point 1 above; point 2 is implied.

src/_pytest/skipping.py

@@ -17,7 +17,7 @@ def pytest_addoption(parser):
         action="store_true",
         dest="runxfail",
         default=False,
-        help="run tests even if they are marked xfail",
+        help="only run xfail tests",
     )
 
     parser.addini(