Silence checkdoc messages about docstrings of internal objects

Internal objects here don’t have and shouldn’t have documentation
strings because:

- There is nothing useful to specify in them.
- They are not a stable API to be used.
- They would pollute the source file and make it larger.

pw-lib.el

@@ -330,4 +330,8 @@ otherwise set the current default sink or source."
 
 (provide 'pw-lib)
 
+;; Local Variables:
+;; checkdoc-force-docstrings-flag: nil
+;; End:
+
 ;;; pw-lib.el ends here