MeshTools: make the generateFlatNormals() docs less handwavy.

If it takes *me* ten minutes to figure out how this needs to be used,
then it probably should be documented.

doc/snippets/MagnumMeshTools.cpp

@@ -92,6 +92,12 @@ std::vector<Vector3> normals;
 std::tie(normalIndices, normals) =
     MeshTools::generateFlatNormals(vertexIndices, positions);
 /* [generateFlatNormals] */
+
+/* [generateFlatNormals-recombine] */
+std::vector<UnsignedInt> indices = MeshTools::combineIndexedArrays(
+    std::make_pair(std::cref(vertexIndices), std::ref(positions)),
+    std::make_pair(std::cref(normalIndices), std::ref(normals)));
+/* [generateFlatNormals-recombine] */
 }
 
 {

src/Magnum/MeshTools/GenerateFlatNormals.h

@@ -43,13 +43,16 @@ namespace Magnum { namespace MeshTools {
 @param positions    Array of vertex positions
 @return Normal indices and vectors
 
-For each face generates one normal vector, removes duplicates before
+All vertices in each triangle face get the same normal vector. Removes
-returning. Example usage:
+duplicates before returning. Example usage:
 
 @snippet MagnumMeshTools.cpp generateFlatNormals
 
-You can then use @ref combineIndexedArrays() to combine normal and vertex array
+This will generate index buffer that's different from the input @p indices
-to use the same indices.
+array, so you'll need to recombine them using @ref combineIndexedArrays() in
+order to have a single index array for both vertices and normals:
+
+@snippet MagnumMeshTools.cpp generateFlatNormals-recombine
 
 @attention The function requires the mesh to have triangle faces, thus index
     count must be divisible by 3.