PolyItem: simplify; disambiguate fcts

GUI/Model/Beam/SourceItems.cpp

@@ -72,7 +72,10 @@ SourceItem::SourceItem()
                      RealLimits::limited(0.0, 1e32), "intensity");
 
     m_azimuthal_angle_item = std::make_unique<BeamDistributionItem>(Units::deg);
-    m_footprint.simpleInit("Type", "Footprint type", FootprintCatalog::Type::Gaussian);
+    m_footprint.simpleInit(
+        "Footprint type",
+        "Model for surface area where scattering takes place (\"beam footprint\")",
+        FootprintCatalog::Type::Gaussian);
 }
 
 void SourceItem::writeTo(QXmlStreamWriter* w) const