docs: kernel: Clarify printk_ratelimit_burst reset behavior

Add clarification that the printk_ratelimit_burst window resets after
printk_ratelimit seconds have elapsed, allowing another burst of
messages to be sent. This helps users understand that the rate limiting
is not permanent but operates in periodic windows.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250714-docs_ratelimit-v1-1-51a6d9071f1a@debian.org

Author: leitao

Documentation/admin-guide/sysctl/kernel.rst

@@ -1111,7 +1111,8 @@ printk_ratelimit_burst
 While long term we enforce one message per `printk_ratelimit`_
 seconds, we do allow a burst of messages to pass through.
 ``printk_ratelimit_burst`` specifies the number of messages we can
-send before ratelimiting kicks in.
+send before ratelimiting kicks in.  After `printk_ratelimit`_ seconds
+have elapsed, another burst of messages may be sent.
 
 The default value is 10 messages.