Add a web reference to arraysize() macro docs.

base/macros.h

Index: base/macros.h
diff --git a/base/macros.h b/base/macros.h
index 5f8d9705d2dd04666f86d13f6a34b49d6b555902..b3e32cac8fdaf488a660d25f667431e65f099b7f 100644
--- a/base/macros.h
+++ b/base/macros.h
@@ -37,10 +37,11 @@
   TypeName() = delete;                           \
   DISALLOW_COPY_AND_ASSIGN(TypeName)
 
-// The arraysize(arr) macro returns the # of elements in an array arr.
-// The expression is a compile-time constant, and therefore can be
-// used in defining new arrays, for example.  If you use arraysize on
-// a pointer by mistake, you will get a compile-time error.
+// The arraysize(arr) macro returns the # of elements in an array arr.  The
+// expression is a compile-time constant, and therefore can be used in defining
+// new arrays, for example.  If you use arraysize on a pointer by mistake, you
+// will get a compile-time error.  For the technical details, refer to
+// http://blogs.msdn.com/b/the1/archive/2004/05/07/128242.aspx.
 
 // This template function declaration is used in defining arraysize.
 // Note that the function doesn't need an implementation, as we only