[libc++][docs] Add missing entry for P3379R0 to 21.rst

P3379R0 was implemented in LLVM 21 (13c464b) but the entry for release note is still missing. A following-up PR (4a509f8) fixed this as drive-by but backport was not accepted.

This patch only adds the missing entry.

Author: frederick-vs-ja

libcxx/docs/ReleaseNotes/21.rst

@@ -53,6 +53,7 @@ Implemented Papers
 - P2711R1: Making multi-param constructors of ``views`` ``explicit`` (`Github <https://github.com/llvm/llvm-project/issues/105252>`__)
 - P2770R0: Stashing stashing ``iterators`` for proper flattening (`Github <https://github.com/llvm/llvm-project/issues/105250>`__)
 - P2655R3: ``common_reference_t`` of ``reference_wrapper`` Should Be a Reference Type (`Github <https://github.com/llvm/llvm-project/issues/105260>`__)
+- P3379R0: Constrain ``std::expected`` equality operators (`Github <https://github.com/llvm/llvm-project/issues/118135>`__)
 
 Improvements and New Features
 -----------------------------