Distant plugin fix and docs update

Author: leroyvn

src/sensors/distant.cpp

@@ -32,9 +32,14 @@ Distant directional sensor (:monosp:`distant`)
      uniformly over the entire scene.
 
 This sensor plugin implements a distant directional sensor which records
-radiation leaving the scene in a given direction. If the ``target`` parameter
-is not set, rays cast by the sensor will be distributed uniformly on the cross
-section of the scene's bounding sphere.
+radiation leaving the scene in a given direction. By default, it records the 
+(spectral) radiant flux per unit solid angle leaving the scene in the specified
+direction (in unit power per unit solid angle). Rays cast by the sensor are 
+distributed uniformly on the cross section of the scene's bounding sphere.
+
+If the ``target`` parameter is not set, the sensor looks at a single point and
+records a (spectral) radiant flux per unit surface area per unit solid angle 
+(in unit power per unit surface area per unit solid angle).
 
 */
 
@@ -116,7 +121,10 @@ MTS_VARIANT class DistantSensor final : public Sensor<Float, Spectrum> {
         }
 
         ray.update();
-        return std::make_pair(ray, wav_weight);
+        return std::make_pair(
+            ray, m_has_target
+                     ? wav_weight
+                     : wav_weight * (math::Pi<Float> * sqr(m_bsphere.radius)));
     }
 
     std::pair<RayDifferential3f, Spectrum> sample_ray_differential(
@@ -153,7 +161,10 @@ MTS_VARIANT class DistantSensor final : public Sensor<Float, Spectrum> {
         ray.has_differentials = false;
 
         ray.update();
-        return std::make_pair(ray, wav_weight);
+        return std::make_pair(
+            ray, m_has_target
+                     ? wav_weight
+                     : wav_weight * (math::Pi<Float> * sqr(m_bsphere.radius)));
     }
 
     /// This sensor does not occupy any particular region of space, return an