gh-148672: Document namespace subpackages inside regular packages

The import system already creates implicit namespace packages for any
subdirectory of a regular package that lacks an ``__init__.py`` file,
per :pep:`420`.  Document this behaviour in Doc/reference/import.rst
so readers do not have to infer it from the PEP and from importlib
internals.

Author: Taeknology

Doc/reference/import.rst

@@ -122,6 +122,12 @@ Importing ``parent.one`` will implicitly execute ``parent/__init__.py`` and
 ``parent.three`` will execute ``parent/two/__init__.py`` and
 ``parent/three/__init__.py`` respectively.
 
+A subdirectory inside a regular package that does not contain an
+``__init__.py`` file is treated as an implicit
+:ref:`namespace package <reference-namespace-package>` (a "namespace
+subpackage") rooted in that parent.  See :pep:`420` for the underlying
+specification.
+
 
 .. _reference-namespace-package:
 
@@ -153,6 +159,12 @@ physically located next to ``parent/two``.  In this case, Python will create a
 namespace package for the top-level ``parent`` package whenever it or one of
 its subpackages is imported.
 
+Namespace packages may also be nested inside a regular package.  When the
+import system searches a regular package's ``__path__`` and encounters a
+subdirectory that does not contain an ``__init__.py`` file, that
+subdirectory becomes a :term:`portion` contributing to a namespace
+subpackage of the enclosing regular package.
+
 See also :pep:`420` for the namespace package specification.