Update radiation.jl

Just correct some comments.

src/radiation.jl

@@ -59,14 +59,14 @@ function thermal_radiation(shape::ShapeModel, emissivities::AbstractVector{<:Rea
         ε = emissivities[i]
         T = temperatures[i]
 
-        ## Direction and distance from the facet to the observer
+        ## Direction and distance from the facet center to the observer
         d̂ = normalize(obs - c)
         d = norm(obs - c)
 
         cosθ = n̂ ⋅ d̂
-        cosθ < 0 && continue  # Ignore the facet if the observer is on the back side
+        cosθ < 0 && continue  # Ignore the facet if the observer is below the horizon.
 
-        ## TO DO: Ray-trace for each facet
+        ## TODO: Ray-trace for each facet
 
         L += ε * σ_SB * T^4 * a * cosθ / (π * d^2)  # π, 2π, or 4π?
     end