Added docs for null_deleter.

doc/core.qbk

@@ -63,7 +63,7 @@ Currently, the Core library contains:
    [`boost::swap`]
  ]
  [
-   [null_deleter]
+   [[link core.null_deleter null_deleter]]
    [`boost::null_deleter`]
  ]
  [
@@ -84,3 +84,4 @@ Currently, the Core library contains:
 [include:core no_exceptions_support.qbk]
 [include:core noncopyable.qbk]
 [include:core swap.qbk]
+[include:core null_deleter.qbk]

doc/null_deleter.qbk

@@ -0,0 +1,22 @@
+[/
+ / Copyright (c) 2014 Andrey Semashev
+ /
+ / Distributed under 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)
+ /]
+
+[section:null_deleter Header <boost/core/null_deleter.hpp>]
+
+The header `<boost/core/null_deleter.hpp>` defines the `boost::null_deleter` function object,
+which can be used as a deleter with smart pointers such as `unique_ptr` or `shared_ptr`. The
+deleter doesn't do anything with the pointer provided upon deallocation, which makes it useful
+when the pointed object is deallocated elsewhere.
+
+Example
+
+    std::shared_ptr< std::ostream > make_stream()
+    {
+        return std::shared_ptr< std::ostream >(&std::cout, boost::null_deleter());
+    }
+
+[endsect]