Clarify that __path__ can't be just any value (GH-6554)

(cherry picked from commit 441d945e)
Co-authored-by: Brett Cannon <brettcannon@users.noreply.github.com>

Doc/reference/import.rst

@@ -616,8 +616,7 @@ the module.
 module.__path__
 ---------------
 
-By definition, if a module has a ``__path__`` attribute, it is a package,
-regardless of its value.
+By definition, if a module has a ``__path__`` attribute, it is a package.
 
 A package's ``__path__`` attribute is used during imports of its subpackages.
 Within the import machinery, it functions much the same as :data:`sys.path`,

Misc/NEWS.d/next/Documentation/2018-04-20-14-09-36.bpo-33276.rA1z_3.rst

+Clarify that the ``__path__`` attribute on modules cannot be just any value.