Add a description for the Renderable base class (#3486)

src/rendering/renderable.cpp

@@ -73,16 +73,27 @@ namespace {
         openspace::properties::Property::Visibility::AdvancedUser
     };
 
+    // This is the base class for all `Renderable` types. These objects are responsible
+    // for rendering the visuals for scene graph nodes in the 3D rendering. Different
+    // `Renderable` types create various kinds of visuals, sometimes based on external
+    // data files or resources (such as images, 3D models, color maps, or datasets).
+    //
+    // A `Renderable` is created by adding it to a scene graph node in an asset. In
+    // general, this places the rendered object at the position of that scene graph node,
+    // unless otherwise specified by the specific `Renderable` type.
+    //
+    // Each `Renderable` type typically also includes a number of settings to alter its
+    // appearance. See the documentation for the individual types for more details.
     struct [[codegen::Dictionary(Renderable)]] Parameters {
         // [[codegen::verbatim(EnabledInfo.description)]]
         std::optional<bool> enabled;
 
         // This value determines the opacity of this renderable. A value of 0 means
-        // completely transparent
+        // completely transparent.
         std::optional<float> opacity [[codegen::inrange(0.0, 1.0)]];
 
         // A single tag or a list of tags that this renderable will respond to when
-        // setting properties
+        // setting properties.
         std::optional<std::variant<std::vector<std::string>, std::string>> tag;
 
         // [[codegen::verbatim(RenderableTypeInfo.description)]]