Added missing copyright comments. Corrected links to the documentation on the comments.

doc/enable_if.qbk

@@ -1,3 +1,16 @@
+[/
+ / Copyright 2003 (c) The Trustees of Indiana University.
+ / Copyright 2014 (c) Glen Fernandes
+ /
+ / Use, modification, and distribution is subject to the Boost Software
+ / License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+ / http://www.boost.org/LICENSE_1_0.txt)
+ /
+ /    Authors: Jaakko Jarvi (jajarvi at osl.iu.edu)
+ /             Jeremiah Willcock (jewillco at osl.iu.edu)
+ /             Andrew Lumsdaine (lums at osl.iu.edu)
+ /]
+
 [section:enable_if enable_if]
 
 [simplesect Authors]