[docs] Add a release note for the removing of -Wreturn-std-move-in-c++11

`-Wreturn-std-move-in-c++11` has been removed in fbee4a0c79.

Reviewed By: aaronpuchert, amccarth

Differential Revision: https://reviews.llvm.org/D97364

clang/docs/ReleaseNotes.rst

@@ -79,6 +79,10 @@ Removed Compiler Flags
 - The clang-cl ``/fallback`` flag, which made clang-cl invoke Microsoft Visual
   C++ on files it couldn't compile itself, has been removed.
 
+- ``-Wreturn-std-move-in-c++11``, which checked whether an entity is affected by
+  `CWG1579 <https://wg21.link/CWG1579>`_ to become implicitly movable, has been
+  removed.
+
 New Pragmas in Clang
 --------------------