Clarify importlib.machinery.PathFinder's difference from what __import__() does.
diff --git a/Doc/library/importlib.rst b/Doc/library/importlib.rst
index 66a5c4e..e5cc27f 100644
--- a/Doc/library/importlib.rst
+++ b/Doc/library/importlib.rst
@@ -477,7 +477,9 @@
This class does not perfectly mirror the semantics of :keyword:`import` in
terms of :data:`sys.path`. No implicit path hooks are assumed for
- simplification of the class and its semantics.
+ simplification of the class and its semantics. This implies that when
+ ``None`` is found in :data:`sys.path_importer_cache` that it is simply
+ ignored instead of implying a default finder.
Only class methods are defined by this class to alleviate the need for
instantiation.