docs: admin-guide: update description for kernel.hotplug sysctl

It's been a few releases since this defaulted to /sbin/hotplug. Update
the text, and include pointers to the two CONFIG_UEVENT_HELPER{,_PATH}
config knobs whose help text could provide more info, but also hint
that the user probably doesn't need to care at all.

Fixes: 7934779a ("Driver-Core: disable /sbin/hotplug by default")
Signed-off-by: NRasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20210420120638.1104016-1-linux@rasmusvillemoes.dkSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/sysctl/kernel.rst

@@ -333,7 +333,12 @@ hotplug
 =======
 
 Path for the hotplug policy agent.
-Default value is "``/sbin/hotplug``".
+Default value is ``CONFIG_UEVENT_HELPER_PATH``, which in turn defaults
+to the empty string.
+
+This file only exists when ``CONFIG_UEVENT_HELPER`` is enabled. Most
+modern systems rely exclusively on the netlink-based uevent source and
+don't need this.
 
 
 hung_task_all_cpu_backtrace