Document EIGEN_NO_DEBUG macro.

I needed some doxygen tricks to get this to work, so it may not be worth it.

Eigen/Core

@@ -32,7 +32,13 @@
 // then include this file where all our macros are defined. It's really important to do it first because
 // it's where we do all the alignment settings (platform detection and honoring the user's will if he
 // defined e.g. EIGEN_DONT_ALIGN) so it needs to be done before we do anything with vectorization.
-#include "src/Core/util/Macros.h"
+#ifndef EIGEN_PARSED_BY_DOXYGEN
+  #include "src/Core/util/Macros.h"
+#else
+  namespace Eigen {  // for some reason Doxygen needs this namespace
+    #include "src/Core/util/Macros.h"
+  }
+#endif
 
 // if alignment is disabled, then disable vectorization. Note: EIGEN_ALIGN is the proper check, it takes into
 // account both the user's will (EIGEN_DONT_ALIGN) and our own platform checks

Eigen/src/Core/util/Macros.h

@@ -109,6 +109,19 @@
 
 #define EIGEN_DEBUG_VAR(x) std::cerr << #x << " = " << x << std::endl;
 
+#ifdef EIGEN_PARSED_BY_DOXYGEN
+  /** \def EIGEN_NO_DEBUG
+    * \ingroup Core_Module 
+    * \brief If defined, Eigen's assertions are disabled.
+    * \details Disabling run-time assertions improves the performance, but it is dangerous because the
+    * assertions guard against programming errors. By default, the EIGEN_NO_DEBUG macro is not defined and
+    * Eigen's run-time assertions are thus enabled. However, if the NDEBUG macro is defined (this is a
+    * standard C++ macro which disables all asserts), then the EIGEN_NO_DEBUG macro will also be defined, and
+    * so Eigen's assertions will also be disabled.
+    */
+  #define EIGEN_NO_DEBUG
+#endif
+
 #ifdef NDEBUG
 # ifndef EIGEN_NO_DEBUG
 #  define EIGEN_NO_DEBUG