This seems like an oversight from

http://bugs.python.org/issue15473

The release notes for 3.3 added a note about this:
"Because None is now inserted into sys.path_importer_cache, if you are clearing out entries in the dictionary of paths that do not have a finder, you will need to remove keys paired with values of None and imp.NullImporter to be backwards-compatible. This will lead to extra overhead on older versions of Python that re-insert None into sys.path_importer_cache where it repesents the use of implicit finders, but semantically it should not change anything."

But the relevant docs for NullImporter (http://docs.python.org/2/library/imp.html#imp.NullImporter) and sys.path_importer_cache (http://docs.python.org/2/library/sys.html#sys.path_importer_cache) did not get updated.

We have verified that the release notes in 3.4 is correct, and NullImporter is never used.

We are however not sure about the correct wording for the documentation fix.