Add extra info from SurfaceTool class page

Added a few sentences about generate_normals() and generate_tangents() taken from the SurfaceTool class page in the docs.

tutorials/content/procedural_geometry/surfacetool.rst

@@ -87,7 +87,13 @@ you want to use unique normals or colors per face instead of per-vertex), you ca
 
     st.deindex()
 
-If you don't add custom normals yourself, you can add them using ``generate_normals()``. The same goes for tangents.
+If you don't add custom normals yourself, you can add them using ``generate_normals()``, which should
+be called after generating geometry and before committing the mesh using ``commit()`` or
+``commit_to_arrays()``. Calling ``generate_normals(true)`` will flip the resulting normals. As a side
+note, ``generate_normals()`` only works if the primitive type is set to ``Mesh.PRIMITIVE_TRIANGLES``.
+
+If you don't add custom tangents, they can be added with ``generate_tangents()``, but it requires
+that each vertex have UVs and normals set already.
 
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