drivers/virtio: Clarify CONFIG_VIRTIO_MEM for unsupported architectures

Let's make it clearer that simply unlocking CONFIG_VIRTIO_MEM on an
architecture is most probably not sufficient to have it working as
expected.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Gavin Shan <gshan@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20220610094737.65254-1-david@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/Kconfig

@@ -123,9 +123,11 @@ config VIRTIO_MEM
 	 This driver provides access to virtio-mem paravirtualized memory
 	 devices, allowing to hotplug and hotunplug memory.
 
-	 This driver was only tested under x86-64 and arm64, but should
-	 theoretically work on all architectures that support memory hotplug
-	 and hotremove.
+	 This driver currently only supports x86-64 and arm64. Although it
+	 should compile on other architectures that implement memory
+	 hot(un)plug, architecture-specific and/or common
+	 code changes may be required for virtio-mem, kdump and kexec to work as
+	 expected.
 
 	 If unsure, say M.