docs: sysctl: Add documentation for /proc/sys/xen/

docs: sysctl: Add documentation for /proc/sys/xen/

Add documentation for the Xen hypervisor sysctl controls in
/proc/sys/xen/balloon/.

Documents the hotplug_unpopulated tunable (available when
CONFIG_XEN_BALLOON_MEMORY_HOTPLUG is enabled) which controls
whether unpopulated memory regions are automatically hotplugged
when the Xen balloon driver needs to reclaim memory.

The documentation is based on source code analysis of
drivers/xen/balloon.c.

Signed-off-by: Shubham Chakraborty <chakrabortyshubham66@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260304150419.16738-1-chakrabortyshubham66@gmail.com>