* add resize(int, NoChange) and resize(NoChange, int)

* add missing assert in resize(int)
* add examples for all resize variants
* expand docs (part of which is from Tim Hutt's e-mail)

doc/snippets/Matrix_resize_int.cpp

@@ -0,0 +1,6 @@
+VectorXd v(10);
+v.resize(3);
+RowVector3d w;
+w.resize(3); // this is legal, but has no effect
+cout << "v: " << v.rows() << " rows, " << v.cols() << " cols" << endl;
+cout << "w: " << w.rows() << " rows, " << w.cols() << " cols" << endl;