some small improvements to the page on functions taking eigen objects.

- make the beginning more precise
 - make the first example be a full selfcontained compiled example, no need for all the others, but having the first one doesn't hurt.

doc/examples/function_taking_eigenbase.cpp

@@ -0,0 +1,18 @@
+#include <iostream>
+#include <Eigen/Core>
+using namespace Eigen;
+
+template <typename Derived>
+void print_size(const EigenBase<Derived>& b)
+{
+  std::cout << "size (rows, cols): " << b.size() << " (" << b.rows()
+            << ", " << b.cols() << ")" << std::endl;
+}
+
+int main()
+{
+    Vector3f v;
+    print_size(v);
+    // v.asDiagonal() returns a 3x3 diagonal matrix pseudo-expression
+    print_size(v.asDiagonal());
+}