Updated docs for release.

Added OSG_USE_DOUBLE_MARTRICES define into include/osg/Matrix to make it more
convinient to switch between single and double matrices.

include/osg/Matrixd

@@ -33,7 +33,7 @@ class SG_EXPORT Matrixd
 
     public:
     
-        typedef float value_type;
+        typedef double value_type;
 
         inline Matrixd() { makeIdentity(); }
         inline Matrixd( const Matrixd& mat) { set(mat.ptr()); }