Clarify how topics.py gets created. (#106121)

When changing docs, it was easy to find text in topics.py, and I
wondered whether I was supposed to edit it.  Thankfully, the top of the
file says it's auto-generated, so I knew I didn't have to edit it. But I
didn't know what started the auto-generation process.

It's part of the release process, so I'll leave a note here for future
editors.

Doc/tools/extensions/pyspecific.py

@@ -559,6 +559,7 @@ class PydocTopicsBuilder(Builder):
         try:
             f.write('# -*- coding: utf-8 -*-\n'.encode('utf-8'))
             f.write(('# Autogenerated by Sphinx on %s\n' % asctime()).encode('utf-8'))
+            f.write('# as part of the release process.\n'.encode('utf-8'))
             f.write(('topics = ' + pformat(self.topics) + '\n').encode('utf-8'))
         finally:
             f.close()