Document enums in Constants.h (bug #248).

To get the links to work, I also had to document the Eigen namespace.
Unfortunately, this means that the word Eigen is linked whenever it appears
in the docs.

Eigen/Core

@@ -185,6 +185,7 @@
 // defined in bits/termios.h
 #undef B0
 
+/** \brief Namespace containing all symbols from the %Eigen library. */
 namespace Eigen {
 
 inline static const char *SimdInstructionSetsInUse(void) {