Cleanup: fix include guard comments and names

libeigen/eigen!2517

doc/snippets/Tutorial_range_for_loop_2d.cpp

@@ -0,0 +1,7 @@
+// SPDX-FileCopyrightText: The Eigen Authors
+// SPDX-License-Identifier: MPL-2.0
+
+Matrix2i A = Matrix2i::Random();
+cout << "Here are the coeffs of the 2x2 matrix A:\n";
+for (auto x : A.reshaped()) cout << x << " ";
+cout << "\n";