Update doc for radiation parameters

Signed-off-by: cwasicki <[email protected]>

proto/frequenz/api/weather/weather.proto

@@ -85,12 +85,16 @@ enum ForecastFeature {
 
   // Amount of solar radiation (shortwave radiation, direct and diffused) that
   // reaches a horizontal plane at the surface of the Earth.
+  // The value is the forecast of the average solar irradiance from the
+  // validity time until the next validity timestamp.
   // Measured in W/m².
   FORECAST_FEATURE_SURFACE_SOLAR_RADIATION_DOWNWARDS = 6;
 
   // Amount of solar radiation (shortwave radiation, direct and diffused) that
   // reaches a horizontal plane at the surface of the Earth minus the amount
   // reflected by the Earth's surface (which is governed by the albedo).
+  // The value is the forecast of the average solar irradiance from the
+  // validity time until the next validity timestamp.
   // Measured in W/m².
   FORECAST_FEATURE_SURFACE_NET_SOLAR_RADIATION = 7;
 }