Document EIGEN_DEFAULT_DENSE_INDEX_TYPE.
Also, expand description of EIGEN_DONT_ALIGN.

Eigen/src/Core/DenseBase.h

diff --git a/Eigen/src/Core/DenseBase.h b/Eigen/src/Core/DenseBase.h
index 496218d..838fa40 100644
--- a/Eigen/src/Core/DenseBase.h
+++ b/Eigen/src/Core/DenseBase.h
@@ -56,7 +56,13 @@
     class InnerIterator;
 
     typedef typename internal::traits<Derived>::StorageKind StorageKind;
-    typedef typename internal::traits<Derived>::Index Index; /**< The type of indices */
+
+    /** \brief The type of indices 
+      * \details To change this, \c \#define the preprocessor symbol \c EIGEN_DEFAULT_DENSE_INDEX_TYPE.
+      * \sa \ref TopicPreprocessorDirectives.
+      */
+    typedef typename internal::traits<Derived>::Index Index; 
+
     typedef typename internal::traits<Derived>::Scalar Scalar;
     typedef typename internal::packet_traits<Scalar>::type PacketScalar;
     typedef typename NumTraits<Scalar>::Real RealScalar;

doc/I14_PreprocessorDirectives.dox

diff --git a/doc/I14_PreprocessorDirectives.dox b/doc/I14_PreprocessorDirectives.dox
index fb83f94..b34fc84 100644
--- a/doc/I14_PreprocessorDirectives.dox
+++ b/doc/I14_PreprocessorDirectives.dox
@@ -22,6 +22,8 @@
    Eigen3; see \ref Eigen2SupportModes.
  - \b EIGEN_DEFAULT_TO_ROW_MAJOR - when defined, the default storage order for matrices becomes row-major
    instead of column-major. Not defined by default.
+ - \b EIGEN_DEFAULT_DENSE_INDEX_TYPE - the type for column and row indices in matrices, vectors and array
+   (DenseBase::Index). Set to \c std::ptrdiff_t by default.
  - \b EIGEN_DEFAULT_IO_FORMAT - the IOFormat to use when printing a matrix if no #IOFormat is specified.
    Defaults to the #IOFormat constructed by the default constructor IOFormat().
  - \b EIGEN_INITIALIZE_MATRICES_BY_ZERO - if defined, all entries of newly constructed matrices and arrays are
@@ -43,7 +45,8 @@
 
 \section TopicPreprocessorDirectivesPerformance Alignment, vectorization and performance tweaking
 
- - \b EIGEN_DONT_ALIGN - disables alignment.
+ - \b EIGEN_DONT_ALIGN - disables alignment completely. Eigen will not try to align its objects and does not
+   expect that any objects passed to it are aligned. This will turn off vectorization. Not defined by default.
  - \b EIGEN_DONT_ALIGN_STATICALLY - disables alignment of arrays on the stack. Not defined by default, unless
    \c EIGEN_DONT_ALIGN is defined.
  - \b EIGEN_DONT_VECTORIZE - disables explicit vectorization when defined. Not defined by default, unless