docs: admin-guide: fix printk_ratelimit explanation

docs: admin-guide: fix printk_ratelimit explanation

The printk_ratelimit value accepts seconds, not jiffies (though it is
converted into jiffies internally). Update documentation to reflect
this.

Also, remove the statement about allowing 1 message in 5 seconds since
bursts up to 10 messages are allowed by default.

Finally, while we are here, mention default value for
printk_ratelimit_burst too.

Signed-off-by: Oleksandr Natalenko <oleksandr@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>