Fix documentation on exportable C# native arrays

It's quite hard to tell what is and isn't permissible from the
current description, and there are both false positives and negatives.

This makes explicit what is and isn't permissible, and suggests a
general workaround when the relevant diagnostic is raised for a native
array type.

Author: esainane

tutorials/scripting/c_sharp/c_sharp_exports.rst

@@ -573,7 +573,13 @@ The default value of Godot dictionaries is null. A different default can be spec
 Exporting C# arrays
 ^^^^^^^^^^^^^^^^^^^
 
-C# arrays can exported as long as the element type is a :ref:`Variant-compatible type <c_sharp_variant_compatible_types>`.
+C# arrays can exported as long as it not multi-dimensional, and the element type is one of the following:
+
+..
+  See Godot.SourceGenerators.MarshalUtils.ConvertManagedTypeToMarshalType for the function which determines what can be marshalled
+* A native C# ``byte``, ``int``/``Int32``, ``Int64``, ``float``, ``double``, or ``string``.
+* Anything derived from ``GodotObject``.
+* Godot's ``Vector2``, ``Vector3``, ``Vector4``, ``Color``, ``StringName``, ``NodePath``, or ``Rid``.
 
 .. code-block:: csharp
 
@@ -583,6 +589,9 @@ C# arrays can exported as long as the element type is a :ref:`Variant-compatible
     [Export]
     public NodePath[] NodePaths { get; set; }
 
+Other element types, even other :ref:`Variant-compatible types <c_sharp_variant_compatible_types>` such as ``Plane`` or ``Transform2D``, will not work.
+If you get a :ref:`GD0102 <doc_c_sharp_diagnostics_gd0102>` error, use a ``Godot.Collections.Array`` instead.
+
 The default value of C# arrays is null. A different default can be specified:
 
 .. code-block:: csharp

tutorials/scripting/c_sharp/diagnostics/GD0102.rst

@@ -1,3 +1,5 @@
+.. _doc_c_sharp_diagnostics_gd0102:
+
 GD0102: The type of the exported member is not supported
 ========================================================