[PATCH] vfb: Document option to enable the driver

The Kconfig help text doesnt mention that in order to load the vfb module, you
have to pass in the parameter vfb_enable=1

Document required module options in Kconfig for loading the vfb module.

Signed-off-by: Antonino Daplas <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>