Added Rationale section from the original docs in Boost.Utility.

doc/addressof.qbk

@@ -22,7 +22,8 @@
 The header `<boost/core/addressof.hpp>` defines the function
 template `boost::addressof`. `boost::addressof(x)` returns the
 address of `x`. Ordinarily, this address can be obtained by
-`&x`, but the unary `&` operator can be overloaded.
+`&x`, but the unary `&` operator can be overloaded. `boost::addressof`
+avoids calling used-defined `operator&()`.
 
 `boost::addressof` was originally contributed by Brad King
 based on ideas from discussion with Doug Gregor.

doc/noncopyable.qbk

@@ -51,6 +51,22 @@ class X: private boost::noncopyable
 
 [endsect]
 
+[section Rationale]
+
+Class noncopyable has protected constructor and destructor members to emphasize
+that it is to be used only as a base class. Dave Abrahams notes concern about
+the effect on compiler optimization of adding (even trivial inline) destructor
+declarations. He says:
+
+["Probably this concern is misplaced, because `noncopyable` will be used mostly
+for classes which own resources and thus have non-trivial destruction semantics.]
+
+With C++2011, using an optimized and trivial constructor and similar destructor
+can be enforced by declaring both and marking them `default`. This is done in
+the current implementation.
+
+[endsect]
+
 [endsect]
 
 [endsect]