drm/vkms: Update VKMS documentation

Small changes in the driver documentation, clarifing the description.

Signed-off-by: Gabriela Bittencourt <gabrielabittencourt00@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191101223735.2425-1-gabrielabittencourt00@gmail.com