Merged changes from trunk: added intrusive_ref_counter.

[SVN r85609]

intrusive_ptr.html

@@ -24,7 +24,9 @@
 			compilers that support argument-dependent lookup, <STRONG>intrusive_ptr_add_ref</STRONG>
 			and <STRONG>intrusive_ptr_release</STRONG> should be defined in the namespace 
 			that corresponds to their parameter; otherwise, the definitions need to go in 
-			namespace <STRONG>boost</STRONG>.</p>
+			namespace <STRONG>boost</STRONG>. The library provides a helper base class template
+			<STRONG><a href="intrusive_ref_counter.html">intrusive_ref_counter</a></STRONG> which may
+			help adding support for <STRONG>intrusive_ptr</STRONG> to user&apos;s types.</p>
 		<p>The class template is parameterized on <b>T</b>, the type of the object pointed 
 			to. <STRONG>intrusive_ptr&lt;T&gt;</STRONG> can be implicitly converted to <STRONG>intrusive_ptr&lt;U&gt;</STRONG>
 			whenever <STRONG>T*</STRONG> can be implicitly converted to <STRONG>U*</STRONG>.</p>