DOC: add moved modules to 1.18 release note

Author: mattip

doc/source/release/1.18.0-notes.rst

@@ -199,6 +199,14 @@ exception will require adaptation, and code that mistakenly called
 ``numpy.nanmax`` and ``numpy.nanmin``.
 (`gh-14841 <https://github.com/numpy/numpy/pull/14841>`__)
 
+Moved modules in ``numpy.random``
+---------------------------------
+As part of the API cleanup, the submodules in ``numpy.random``
+``bit_generator``, ``philox``, ``pcg64``, ``sfc64, ``common``, ``generator``,
+and ``bounded_integers`` were moved to ``_bit_generator``, ``_philox``,
+``_pcg64``, ``_sfc64, ``_common``, ``_generator``, and ``_bounded_integers``
+respectively to indicate that they are not part of the public interface.
+
 
 C API changes
 =============