[libc++] Clarify the release note for uncaught_exception removal and deprecation (#123118)

The release note did not clearly mention that std::uncaught_exception
had been removed in C++20.

Author: ldionne

libcxx/docs/ReleaseNotes/20.rst

@@ -55,7 +55,8 @@ Improvements and New Features
 - The ``_LIBCPP_ENABLE_CXX20_REMOVED_TEMPORARY_BUFFER`` macro has been added to make ``std::get_temporary_buffer`` and
   ``std::return_temporary_buffer`` available.
 
-- The ``_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION`` macro has been added to make ``std::uncaught_exception``
+- The ``std::uncaught_exception`` function was marked as deprecated since C++17 and removed since C++20. The
+  ``_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION`` macro has been added to make ``std::uncaught_exception``
   available in C++20 and later modes.
 
 - The internal structure ``__compressed_pair`` has been replaced with ``[[no_unique_address]]``, resulting in reduced
@@ -69,12 +70,12 @@ Improvements and New Features
 - The ``_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY`` ABI configuration was added, which allows storing valid bounds
   in ``std::array::iterator`` and detecting OOB accesses when the appropriate hardening mode is enabled.
 
-- The ``input_iterator``-pair overload of ``void assign(InputIt, InputIt)`` has been optimized for ``std::vector``, 
-  resulting in a performance improvement of up to 2x for trivial element types (e.g., ``std::vector<int>``), and up 
+- The ``input_iterator``-pair overload of ``void assign(InputIt, InputIt)`` has been optimized for ``std::vector``,
+  resulting in a performance improvement of up to 2x for trivial element types (e.g., ``std::vector<int>``), and up
   to 3.4x for non-trivial element types (e.g., ``std::vector<std::vector<int>>``).
 
-- The ``input_iterator``-pair overload of ``iterator insert(const_iterator, InputIt, InputIt)`` has been optimized 
-  for ``std::vector``, resulting in a performance improvement of up to 10x for ``std::vector<int>``, and up to 2.3x 
+- The ``input_iterator``-pair overload of ``iterator insert(const_iterator, InputIt, InputIt)`` has been optimized
+  for ``std::vector``, resulting in a performance improvement of up to 10x for ``std::vector<int>``, and up to 2.3x
   for ``std::vector<std::vector<int>>``.
 
 - On Windows, ``<system_error>``'s ``std::system_category`` is now distinct from ``std::generic_category``. The behavior