Documentation: doc-guide: use '%' constant indicator in Return: examples

Use the 'constant' indicator '%' in the examples for the
Return: values syntax. This can help encourage people to use it.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/lkml/20221121154358.36856ca6@gandalf.local.home/


Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230703232030.8223-1-rdunlap@infradead.org