virtio: add help to CONFIG_VIRTIO option.

Trying to enable a virtio driver (eg CONFIG_VIRTIO_BLK) is painful
because it depends on CONFIG_VIRTIO.  CONFIG_VIRTIO doesn't tell you
how to turn it on (it's selected from anything which provides a virtio
bus).

This patch at least adds some documentation, visible in menuconfig, as
a hint.

Reported-by: Kent Overstreet <koverstreet@google.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/Kconfig

@@ -1,8 +1,10 @@
-# Virtio always gets selected by whoever wants it.
 config VIRTIO
 	tristate
+	---help---
+	  This option is selected by any driver which implements the virtio
+	  bus, such as CONFIG_VIRTIO_PCI, CONFIG_VIRTIO_MMIO, CONFIG_LGUEST,
+	  CONFIG_RPMSG or CONFIG_S390_GUEST.
 
-# Similarly the virtio ring implementation.
 config VIRTIO_RING
 	tristate
 	depends on VIRTIO