many small fixes and documentation improvements,

this should be alpha5.

doc/snippets/MatrixBase_inverse.cpp

@@ -0,0 +1,7 @@
+Matrix2d m = Matrix2d::random();
+cout << "Here is the matrix m:" << endl << m << endl;
+Matrix2d::InverseType m_inv = m.inverse();
+if(m_inv.exists())
+  cout << "m is invertible, and its inverse is:" << endl << m_inv << endl;
+else
+  cout << "m is not invertible." << endl;