[Docs] Add/update release notes for D71913 (LTO WPD changes)

This adds documentation for the options added / changed by D71913, which
enabled aggressive WPD under LTO. The lld release notes already
mentioned it, but I expanded the note.

Differential Revision: https://reviews.llvm.org/D86958

Author: teresajohnson

clang/docs/ReleaseNotes.rst

@@ -306,7 +306,8 @@ Modified Compiler Flags
 - -mcpu is now supported for RISC-V, and recognises the generic-rv32,
   rocket-rv32, sifive-e31, generic-rv64, rocket-rv64, and sifive-u54 target
   CPUs.
-
+- ``-fwhole-program-vtables`` (along with ``-flto*``) now prepares all classes for possible whole program visibility if specified during the LTO link.
+  (`D71913 <https://reviews.llvm.org/D71913>`_)
 
 New Pragmas in Clang
 --------------------

lld/docs/ReleaseNotes.rst

@@ -26,7 +26,7 @@ ELF Improvements
 
 * ``--lto-emit-asm`` is added to emit assembly output for debugging purposes.
   (`D77231 <https://reviews.llvm.org/D77231>`_)
-* ``--lto-whole-program-visibility`` is added to support LTO whole-program devirtualization.
+* ``--lto-whole-program-visibility`` is added to specify that classes have hidden LTO visibility in LTO and ThinLTO links of source files compiled with ``-fwhole-program-vtables``. See `LTOVisibility <https://clang.llvm.org/docs/LTOVisibility.html>`_ for details.
   (`D71913 <https://reviews.llvm.org/D71913>`_)
 * ``--print-archive-stats=`` is added to print the number of members and the number of fetched members for each archive.
   The feature is similar to GNU gold's ``--print-symbol-counts=``.

llvm/docs/ReleaseNotes.rst

@@ -333,6 +333,12 @@ Changes to the Debug Info
   passed to the callee. The feature improves the debugging user experience when
   debugging optimized code.
 
+Changes to the Gold Plugin
+--------------------------
+
+* ``--plugin-opt=whole-program-visibility`` is added to specify that classes have hidden LTO visibility in LTO and ThinLTO links of source files compiled with ``-fwhole-program-vtables``. See `LTOVisibility <https://clang.llvm.org/docs/LTOVisibility.html>`_ for details.
+  (`D71913 <https://reviews.llvm.org/D71913>`_)
+
 Changes to the LLVM tools
 ---------------------------------