[libc++] Add Implemented Papers section

[Mordante]

I'm not convinced this change is really needed, but I'm not objecting either.
However when we move in this direction, lets move all paper related changes to this section.
(I expect some thing migh be ambiguous, but I marked the clear ones.)

libcxx/docs/ReleaseNotes.rst

47

Please move these to the new section too. Just move them as-is.

76–77

Please move these to the new section too. Please leave the additional information only strip the Implemented.

[philnik]

• Address comments

[Mordante]

LGTM, but please give other reviewers a bit more time to have a look.

libcxx/docs/ReleaseNotes.rst

78

Actually I meant to move this entire paragraph to Implemented Papers, but this is fine by me too.

libcxx/docs/ReleaseNotes.rst

 Note that if you are reading this file from a Git checkout or the
 main Libc++ web page, this document applies to the *next* release, not
 the current one. To see the release notes for a specific release, please
 see the `releases page <https://llvm.org/releases/>`_.
 
 What's New in Libc++ 15.0.0?
 ============================
 
-New Features
-------------
-
-- Implemented P0627R6 (Function to mark unreachable code)
-
-- Implemented P1165R1 (Make stateful allocator propagation more consistent for ``operator+(basic_string)``)
+Implemented Papers
+------------------
 
-- Implemented P0674R1 (Support arrays in ``make_shared`` and ``allocate_shared``)
-
-- Implemented P0980R1 (Making ``std::string`` constexpr)
+- P0627R6 (Function to mark unreachable code)
+- P1165R1 (Make stateful allocator propagation more consistent for ``operator+(basic_string)``)
+- P0674R1 (Support arrays in ``make_shared`` and ``allocate_shared``)
+- P0980R1 (Making ``std::string`` constexpr)
+- P2216R3 (std::format improvements)
 
 - Marked the following papers as "Complete" (note that some of those might have
   been implemented in a previous release but not marked as such):
 
     - P1207R4 (Movability of Single-pass Iterators);
     - P1474R1 (Helpful pointers for ``ContiguousIterator``);
     - P1522R1 (Iterator Difference Type and Integer Overflow);
     - P1523R1 (Views and Size Types);
     - P1456R1 (Move-only views);
     - P1870R1 (``forwarding-range`` is too subtle);
     - P1878R1 (Constraining Readable Types);
     - P1970R2 (Consistency for ``size()`` functions: Add ``ranges::ssize``);
     - P1983R0 (Wording for GB301, US296, US292, US291, and US283).
 
+New Features
+------------
+
 - `pop_heap` now uses an algorithm known as "bottom-up heapsort" or
   "heapsort with bounce" to reduce the number of comparisons, and rearranges
   elements using move-assignment instead of `swap`.
 
 - Libc++ now supports a variety of assertions that can be turned on to help catch
   undefined behavior in user code. This new support is now separate from the old
   (and incomplete) Debug Mode. Vendors can select whether the library they ship
   should include assertions or not by default. For details, see
   :ref:`the documentation <assertions-mode>` about this new feature.
 
 - The implementation of the function ``std::to_chars`` for integral types has
   moved from the dylib to the header. This means the function no longer has a
   minimum deployment target.
 
-- Implemented P2216R3 (std::format improvements). The format functions
-  (``std::format``, ``std::format_to``, ``std::format_to_n``, and
+- The format functions (``std::format``, ``std::format_to``, ``std::format_to_n``, and
   ``std::formatted_size``) now validate the format string at compile time.
   When the format string is invalid this will make the code ill-formed instead
   of throwing an exception at run-time.  (This does not affect the ``v``
   functions.)
 
 
 API Changes
 -----------