Update changelog with #248

Author: igaztanaga

doc/container.qbk

@@ -1424,6 +1424,7 @@ use [*Boost.Container]? There are several reasons for that:
 [section:release_notes_boost_1_90_00 Boost 1.90 Release]
 
 * Fixed bugs/issues:
+   * [@https://github.com/boostorg/container/issues/248 GitHub #248: ['"flat_map slow insertion introduced in boost-1.80.0"]].
    * [@https://github.com/boostorg/container/pull/294 GitHub #294: ['"CMake: Add option to use header-only Boost::container"]].
    * [@https://github.com/boostorg/container/issues/300 GitHub #300: ['"Warnings when building with clang-20"]].
    * [@https://github.com/boostorg/container/pull/305 GitHub #305: ['"Warnings with -Wstrict-prototypes"]].