[libc++][doc] Update the release notes.

[Mordante]

Summary

Fixes the incorrect removal version of the experimental coroutine
header.

[Mordante]

libcxx/docs/ReleaseNotes.rst

170

Is this intended to be 15 or 16?

184

Is this intended to be 15 or 16?

[ldionne]

libcxx/docs/ReleaseNotes.rst

170

15, this is OK too.

184

That's 15, this is OK. Our policy of 2 releases is for TSes only.

[Mordante]

Landed as https://reviews.llvm.org/rG5f26d8636f506b487962cd2a9b0e32940e93fa9b

libcxx/docs/ReleaseNotes.rst

- C++20 requires that ``std::basic_string::reserve(n)`` never reduce the capacity
   but specifically permits it to reduce capacity. To avoid breaking existing code
   assuming that ``std::basic_string::reserve()`` will shrink, libc++ maintains
   the behavior to shrink, even though that makes ``std::basic_string::reserve()`` not
   a synonym for ``std::basic_string::reserve(0)`` in any Standard mode anymore.
 
 - The ``<experimental/coroutine>`` header is deprecated, as is any
   use of coroutines without C++20. Use C++20's ``<coroutine>`` header
   instead. The ``<experimental/coroutine>`` header will be removed
-  in LLVM 15.
+  in LLVM 16.
 
 - ``_VSTD`` is now an alias for ``std`` instead of ``std::_LIBCPP_ABI_NAMESPACE``.
   This is technically not a functional change, except for folks that might have been
   using ``_VSTD`` in creative ways (which has never been officially supported).
 
 ABI Changes
 -----------
 
 - The C++17 variable templates ``is_error_code_enum_v`` and
   ``is_error_condition_enum_v`` are now of type ``bool`` instead of ``size_t``.
 
 - The C++03 emulation type for ``std::nullptr_t`` has been removed in favor of
   using ``decltype(nullptr)`` in all standard modes. This is an ABI break for
   anyone compiling in C++03 mode and who has ``std::nullptr_t`` as part of their
   ABI. However, previously, these users' ABI would be incompatible with any other
   binary or static archive compiled with C++11 or later. If you start seeing linker
   errors involving ``std::nullptr_t`` against previously compiled binaries, this may
   be the cause. You can define the ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` macro
   to return to the previous behavior. That macro will be removed in LLVM 15. Please
   comment `on D109459 <https://reviews.llvm.org/D109459>`_ if you are broken by this change
   and need to define the macro.
 
 - On Apple platforms, ``std::random_device`` is now implemented on top of ``arc4random()``
   instead of reading from ``/dev/urandom``. Any implementation-defined token used when
   constructing a ``std::random_device`` will now be ignored instead of interpreted as a
   file to read entropy from.
 
 - ``std::lognormal_distribution::param_type`` used to store a data member of type
   ``std::normal_distribution``; now this member is stored in the ``lognormal_distribution``
   class itself, and the ``param_type`` stores only the mean and standard deviation,
   as required by the Standard. This changes ``sizeof(std::lognormal_distribution::param_type)``.
   You can define the ``_LIBCPP_ABI_OLD_LOGNORMAL_DISTRIBUTION`` macro to return to the
   previous behavior. That macro will be removed in LLVM 15. Please comment
   `on PR52906 <https://llvm.org/PR52906>`_ if you are broken by this change and need to
   define the macro.
 
 Build System Changes
 --------------------
 
 - Building the libc++ shared or static library requires a C++ 20 capable compiler.
   Consider using a Bootstrapping build to build libc++ with a fresh Clang if you