Update release notes for Boost 1.91

Author: igaztanaga

doc/container.qbk

@@ -1434,6 +1434,12 @@ use [*Boost.Container]? There are several reasons for that:
 
 [section:release_notes Release Notes]
 
+[section:release_notes_boost_1_91_00 Boost 1.91 Release]
+
+* Implemented heterogeneous overloads from C++03 ([@http://wg21.link/p2077r3 P2077]) and C++26 ([@http://wg21.link/P2363 P2363]).
+
+[endsect]
+
 [section:release_notes_boost_1_90_00 Boost 1.90 Release]
 
 * Reimplemented [classref boost::container::deque]. The original implementation was based on