Change realease notes

Author: grigorypas

clang/docs/ReleaseNotes.rst

@@ -308,6 +308,8 @@ New Compiler Flags
 
 - New option ``-ftime-report-json`` added which outputs the same timing data as ``-ftime-report`` but formatted as JSON.
 
+- New option ``-Wnrvo`` added and disabled by default to warn about missed NRVO opportunites.
+
 Deprecated Compiler Flags
 -------------------------