Document where mesh transforms are saved, fixes #1894

PiperOrigin-RevId: 662836678
Change-Id: I060e3b1f29e82c6c3c188e730657cda7b99f63e9

doc/XMLreference.rst

@@ -1106,7 +1106,8 @@ to convert to one of the other supported formats.
 
 .. _legacy-msh-docs:
 
-MSH file format
+.. collapse:: Legacy MSH file format
+
    The binary MSH file starts with 4 integers specifying the number of vertex positions (nvertex), vertex normals
    (nnormal), vertex texture coordinates (ntexcoord), and vertex indices making up the faces (nface), followed by the
    numeric data. nvertex must be at least 4. nnormal and ntexcoord can be zero (in which case the corresponding data is
@@ -1190,7 +1191,8 @@ The full list of processing steps applied by the compiler to each mesh is as fol
    normals. If sharp edges are encountered, the renderer uses the face normals to preserve the visual information about
    the edge, unless :ref:`smoothnormal<asset-mesh-smoothnormal>` is true.
    Note that normals cannot be provided with STL meshes;
-#. Scale, translate and rotate the vertices and normals, re-normalize the normals in case of scaling;
+#. Scale, translate and rotate the vertices and normals, re-normalize the normals in case of scaling. Save these
+   transformations in ``mjModel.mesh_{pos, quat, scale}``.
 #. Construct the convex hull if specified;
 #. Find the centroid of all triangle faces, and construct the union-of-pyramids representation. Triangles whose area is
    too small (below the :ref:`mjMINVAL <glNumeric>` value of 1E-14) result in compile error;