Update glossary entry for namespace packages with pep 420 reference

Author: stevepiercy

source/glossary.rst

@@ -122,12 +122,16 @@ Glossary
     Implicit Namespace Package
     Native Namespace Package
 
-        Native namespace and implicit namespace packages are two names for the same thing.
+        "Native namespace packages" and "implicit namespace packages" are two names for
+        the same type of namespace package. It's defined in :pep:`420`, and is available
+        in Python 3.3 and later. 
+
         The term "native" derives from the built-in feature of Python to create packages
         without installing an extra package, such as setuptools, from outside the standard
         library. The term "implicit" derives from the fact that you don't have to explicitly
-        declare the namespace package in a :file:`__init__.py` file. This guide uses these
-        names interchangeably. 
+        declare the namespace package in a :file:`__init__.py` file.
+
+		This guide uses these names interchangeably.
 
     Import Package