doc(layers): improve raster layers doc

docs/tutorials/Fundamentals.md

@@ -76,6 +76,8 @@ Some point clouds formats such as Potree, Las and Entwine already have parsers d
 | :---: | :---: | :---: |
 | A simple `ColorLayer` is displayed | An `ElevationLayer` is added to represent terrain elevation | A `GeometryLayer` is added to model the buildings |
 
+Note that `{@link ColorLayer}` and `{@link ElevationLayer}` don't have their own geometry and are always *attached* to a `{@link GeometryLayer}` (generally the layer representing the geometry of the globe or of the plane). `{@link ColorLayer}` are projected onto this `{@link GeometryLayer}` and `{@link ElevationLayer}` are used to read elevation and to apply it to a `{@link GeometryLayer}`.
+
 ***
 
 ## Style