printk-formats.txt: Add examples for %pF and %pS usage

Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>