hugo ref beam: explain effect of intensity on noise (#873)

hugo/content/ref/instr/beam/_index.md

@@ -19,12 +19,16 @@ For the glancing angle $\alpha\_\text{i}$, use unit multiplier `*deg`.
 The azimuthal angle $\varphi\_\text{i}$ can be omitted,
 as in most applications the default value 0 is pertinent.
 
-After creation of a beam, special properties can be set with
+After creation of a beam, properties can be set with
 ```python
+beam.setIntensity(intensity)
 beam.setFootprintFactor(footprint)
 beam.setPolarization(polarization)
 ```
 For the arguments, see [footprint](footprint) and [polarization](../pol).
 
+The beam intensity may appear inconsequential for specular simulations
+because resulting reflectivities are normalized.
+However, the intensity does influence the simulated [noise](/ref/sim/setup/bg).
 
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