many small fixes and documentation improvements,

this should be alpha5.

doc/snippets/MatrixBase_quickInverse.cpp

@@ -0,0 +1,5 @@
+Matrix4d m = Matrix4d::zero();
+m.part<Eigen::Upper>().setOnes();
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "We know for sure that it is invertible." << endl;
+cout << "Here is its inverse:" << m.quickInverse() << endl;