printk-formats.txt: Better describe the difference between %pS and %pF

Sometimes people seems unclear when to use the %pS or %pF printk format.
For example, see commit 51d96dc2

 ("random: fix warning message on ia64
and parisc") which fixed such a wrong format string.

The documentation should be more clear about the difference.

Signed-off-by: Helge Deller <deller@gmx.de>
[pmladek@suse.com: Restructure the entire section]
Signed-off-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>