Update documentation of `DfpAds`

[asimqt]

Documentation of DfpAds not talking about viewPortSizeMapping which causes the front end to show both mobile and desktop ads in all devices.

const viewPortSizeMappingHorizontal = [ {viewport: [980, 690], sizes:[[728, 90]]},
                              {viewport: [320, 0], sizes:[[320, 50]]},
                              {viewport: [0, 0], sizes:[[320, 50]]}];
export const AD_CONFIG = {
   "Horizontal-Ad": { 
     adUnit: "Madrid_Horizontal_Responsive",
     sizes: [[728, 90], [320, 50]],
     viewPortSizeMapping: viewPortSizeMappingHorizontal
   }
   ...
}