Doc: update term "namespace package" (GH-129251)

(cherry picked from commit e65e9f9)
Co-authored-by: Inada Naoki <[email protected]>

Author: miss-islington

Doc/glossary.rst

@@ -873,11 +873,16 @@ Glossary
       modules, respectively.
 
    namespace package
-      A :pep:`420` :term:`package` which serves only as a container for
-      subpackages.  Namespace packages may have no physical representation,
+      A :term:`package` which serves only as a container for subpackages.
+      Namespace packages may have no physical representation,
       and specifically are not like a :term:`regular package` because they
       have no ``__init__.py`` file.
 
+      Namespace packages allow several individually installable packages to have a common parent package.
+      Otherwise, it is recommended to use a :term:`regular package`.
+
+      For more information, see :pep:`420` and :ref:`reference-namespace-package`.
+
       See also :term:`module`.
 
    nested scope

Doc/reference/import.rst

@@ -123,6 +123,8 @@ Importing ``parent.one`` will implicitly execute ``parent/__init__.py`` and
 ``parent/three/__init__.py`` respectively.
 
 
+.. _reference-namespace-package:
+
 Namespace packages
 ------------------