added docs about the `no:` syntax for the -p option

--HG--
branch : no-p-option

_pytest/helpconfig.py

@@ -12,7 +12,9 @@ def pytest_addoption(parser):
             help="show help message and configuration info")
     group._addoption('-p', action="append", dest="plugins", default = [],
                metavar="name",
-               help="early-load given plugin (multi-allowed).")
+               help="early-load given plugin (multi-allowed). "
+                    "To avoid loading of plugins, use the `no:` prefix, e.g. "
+                    "`no:doctest`.")
     group.addoption('--traceconfig', '--trace-config',
                action="store_true", default=False,
                help="trace considerations of conftest.py files."),

doc/en/usage.txt

@@ -153,6 +153,17 @@ for example ``-x`` if you only want to send one particular failure.
 
 Currently only pasting to the http://bpaste.net service is implemented.
 
+Disabling plugins
+-----------------
+
+To disable loading specific plugins at invocation time, use the ``-p`` option
+together with the prefix ``no:``.
+
+Example: to disable loading the plugin ``doctest``, which is responsible for
+executing doctest tests from text files, invoke py.test like this::
+
+    py.test -p no:doctest
+
 .. _`pytest.main-usage`:
 
 Calling pytest from Python code