Docs: add section on resolving the aliasing issue.

doc/snippets/TopicAliasing_block.cpp

@@ -1,5 +1,7 @@
 MatrixXi mat(3,3); 
 mat << 1, 2, 3,   4, 5, 6,   7, 8, 9;
 cout << "Here is the matrix mat:\n" << mat << endl;
+
+// This assignment shows the aliasing problem
 mat.bottomRightCorner(2,2) = mat.topLeftCorner(2,2);
 cout << "After the assignment, mat = \n" << mat << endl;