Document the import semantics that if None is found in sys.modules for a module then ImportError is raised.

Doc/reference/simple_stmts.rst

@@ -668,7 +668,9 @@ Once the name of the module is known (unless otherwise specified, the term
 "module" will refer to both packages and modules), searching
 for the module or package can begin. The first place checked is
 :data:`sys.modules`, the cache of all modules that have been imported
-previously. If the module is found there then it is used in step (2) of import.
+previously. If the module is found there then it is used in step (2) of import
+unless :keyword:`None` is found in :data:`sys.modules`, in which case
+:exc:`ImportError` is raised.
 
 .. index::
     single: sys.meta_path

Misc/NEWS

@@ -174,6 +174,9 @@ Build
 Documenation
 ------------
 
+- Document that importing a module that has None in sys.modules triggers an
+  ImportError.
+
 - Issue #6556: Fixed the Distutils configuration files location explanation
   for Windows.