Added docs for passenv PYTHONPATH semantics

Author: jarshwah

doc/config.txt

@@ -222,6 +222,12 @@ Complete list of settings that you can put into ``testenv*`` sections:
    invocation environment) can define additional space-separated variable
    names that are to be passed down to the test command environment.
 
+   .. versionchanged:: 2.7
+
+   ``PYTHONPATH`` will be passed down if explicitly defined. If ``PYTHONPATH``
+   exists in the host environment but is **not** declared in ``passenv`` a
+   warning will be emitted.
+
 .. confval:: recreate=True|False(default)
 
     Always recreate virtual environment if this option is True.