[libc++] Add a release note about multi{map,set}::find not returning the first element anymore (#155252)

We've modified the algorithm of `__tree::find` in #152370, which can
change the return value. Since we're always returned the lower bound
before some users started relying on it. This patch adds a release note
so users are aware that this might break their code.

Author: philnik777

libcxx/docs/ReleaseNotes/22.rst

@@ -62,6 +62,8 @@ Deprecations and Removals
 Potentially breaking changes
 ----------------------------
 
+- The algorithm for ``multi{map,set}::find`` has been modified such that it doesn't necessarily return an iterator to the first equal element in the container. This was never guaranteed by the Standard, but libc++ previously happened to always return the first equal element, like other implementations do. Starting with this release, code relying on the first element being returned from ``find`` will be broken, and ``lower_bound`` or ``equal_range`` should be used instead.
+
 Announcements About Future Releases
 -----------------------------------