reference: add note that `pythonpath` does not affect `-p`

Fix #11118.

doc/en/reference/reference.rst

@@ -1703,6 +1703,11 @@ passed multiple times. The expected format is ``name=value``. For example::
         [pytest]
         pythonpath = src1 src2
 
+   .. note::
+
+        ``pythonpath`` does not affect some imports that happen very early,
+        most notably plugins loaded using the ``-p`` command line option.
+
 
 .. confval:: required_plugins