add snippet for sub/super diagonal

fix a few doc issues

doc/snippets/MatrixBase_diagonal_int.cpp

@@ -0,0 +1,5 @@
+Matrix4i m = Matrix4i::Random();
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "Here are the coefficients on the 1st super-diagonal and 2nd sub-diagonal of m:" << endl
+     << m.diagonal<1>().transpose() << endl
+     << m.diagonal<-2>().transpose() << endl;