Add technical definition for manifoldness

Author: tetrapod00

tutorials/3d/csg_tools.rst

@@ -91,15 +91,26 @@ The :ref:`CSGPolygon3D <class_CSGPolygon3D>` node extrude along a Polygon drawn
 Custom meshes
 ~~~~~~~~~~~~~
 
-Any mesh can be used for :ref:`CSGMesh3D <class_CSGMesh3D>`; the mesh can be
-modeled in other software and imported into Godot. Multiple materials are
-supported. There are some restrictions for geometry:
+Custom meshes can be used for :ref:`CSGMesh3D <class_CSGMesh3D>` as long as the
+mesh is *manifold*. The mesh can be modeled in other software and imported into
+Godot. Multiple materials are supported. A manifold mesh has some restrictions
+for geometry:
 
 - it must be closed,
 - it must not self-intersect,
 - it must not contain internal faces,
 - every edge must connect to only two other faces.
 
+Godot uses the `manifold <https://github.com/elalish/manifold>`__ library to
+implement CSG meshes. The technical definition of "manifold" used by Godot is
+the following, adapted from that library's `definition of "manifold"
+<https://github.com/elalish/manifold/wiki/Manifold-Library#manifoldness-definition>`__:
+
+  Every edge of every triangle must contain the same two vertices (by index) as
+  exactly one other triangle edge, and the start and end vertices must switch
+  places between these two edges. The triangle vertices must appear in clockwise
+  order when viewed from the outside of the Godot Engine manifold mesh.
+
 .. image:: img/csg_custom_mesh.png
 
 CSGCombiner3D