release/20.x: [libc++] Add missing release note for LLVM 20 about zip_view (#134144)

libcxx/docs/ReleaseNotes/20.rst

@@ -162,6 +162,12 @@ Deprecations and Removals
 
 - Non-conforming extension ``packaged_task::result_type`` is deprecated. It will be removed in LLVM 21.
 
+- The changes for ``ranges::zip_view`` from `P2165R4 <https://wg21.link/P2165R4>`_ have been implemented. This can
+  lead to code assuming that ``zip_view`` produces ``std::pair`` to stop compiling now that it produces ``std::tuple``.
+  The cases are rare since ``tuple`` and ``pair`` are compatible for the most part, but this can lead to code that
+  was previously accepted now being rejected. This is necessary for libc++ to be conforming, so we don't provide any
+  way to opt-out of that behavior.
+
 Upcoming Deprecations and Removals
 ----------------------------------
 
@@ -205,3 +211,8 @@ ABI Affecting Changes
 
 - The localization support base API has been reimplemented, leading to different functions being exported from the
   libc++ built library on Windows and Windows-like platforms.
+
+- The changes for ``ranges::zip_view`` from `P2165R4 <https://wg21.link/P2165R4>`_ have been implemented. This changes
+  the element type of ``zip_view`` from a ``std::pair`` to a ``std::tuple`` in some cases. This is technically an ABI
+  break, however since ``zip_view`` is generally not an ABI sensitive type, we don't expect users to encounter any
+  issues and we don't provide a way to change this behavior, which would make libc++ non-conforming.