Add NEWS.md entry for new polly command-line flag (#18159). (#21841)

NEWS.md

@@ -498,6 +498,14 @@ Deprecated or removed
   * `bitbroadcast` has been deprecated in favor of `broadcast`, which now produces a
     `BitArray` instead of `Array{Bool}` for functions yielding a boolean result ([#19771]).
 
+Command-line option changes
+---------------------------
+
+  * In `polly` builds (`USE_POLLY := 1`), the new flag `--polly={yes|no}` controls whether
+    `@polly` declarations are respected. (With `--polly=no`, `@polly` declarations are
+    ignored.) This flag is also available in non-`polly` builds (`USE_POLLY := 0`),
+    but has no effect ([#18159]).
+
 <!--- generated by NEWS-update.jl: -->
 [#265]: https://github.com/JuliaLang/julia/issues/265
 [#4615]: https://github.com/JuliaLang/julia/issues/4615
@@ -524,6 +532,7 @@ Deprecated or removed
 [#17758]: https://github.com/JuliaLang/julia/issues/17758
 [#17785]: https://github.com/JuliaLang/julia/issues/17785
 [#18050]: https://github.com/JuliaLang/julia/issues/18050
+[#18159]: https://github.com/JuliaLang/julia/issues/18159
 [#18251]: https://github.com/JuliaLang/julia/issues/18251
 [#18330]: https://github.com/JuliaLang/julia/issues/18330
 [#18339]: https://github.com/JuliaLang/julia/issues/18339