Updated drm documentation to include desscription of aspect ratio property.

v2: Updated aspect ratio specific documentation on top of the HTML table created.
Signed-off-by: NVandana Kannan <vandana.kannan@intel.com>
Cc: Sagar Kamble <sagar.a.kamble@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: NSagar Kamble <sagar.a.kamble@intel.com>
Acked-by: NDave Airlie <airlied@gmail.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>