xen: expand BALLOON_MEMORY_HOTPLUG description

To mention it's also useful for PVH or HVM domains that require
mapping foreign memory or grants.

[boris: "non PV" instead of "translated" at Juergen's request]
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Link: https://lore.kernel.org/r/20200324150015.50496-1-roger.pau@citrix.comReviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/Kconfig

@@ -19,6 +19,9 @@ config XEN_BALLOON_MEMORY_HOTPLUG
 	  It is very useful on critical systems which require long
 	  run without rebooting.
 
+	  It's also very useful for non PV domains to obtain unpopulated physical
+	  memory ranges to use in order to map foreign memory or grants.
+
 	  Memory could be hotplugged in following steps:
 
 	    1) target domain: ensure that memory auto online policy is in