fix issue472: clarify that ``pytest.config.getvalue()`` cannot work

if it's triggered ahead of command line parsing.

CHANGELOG

@@ -18,6 +18,9 @@ NEXT (2.6)
 - fix issue490: include pytest_load_initial_conftests in documentation
   and improve docstring.
 
+- fix issue472: clarify that ``pytest.config.getvalue()`` cannot work
+  if it's triggered ahead of command line parsing.
+
 
 2.5.2
 -----------------------------------

doc/en/skipping.txt

@@ -286,4 +286,9 @@ The equivalent with "boolean conditions" is::
     def test_function(...):
         pass
 
+.. note::
 
+    You cannot use ``pytest.config.getvalue()`` in code
+    imported before py.test's argument parsing takes place.  For example,
+    ``conftest.py`` files are imported before command line parsing and thus
+    ``config.getvalue()`` will not execute correctly.