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

NEWS.md

@@ -470,6 +470,12 @@ 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
+---------------------------
+
+  * The new flag `--polly={yes|no}` controls whether `@polly` declarations are
+    respected. (With `--polly=no`, `@polly` declarations are ignored.) ([#18159]).
+
 <!--- generated by NEWS-update.jl: -->
 [#265]: https://github.com/JuliaLang/julia/issues/265
 [#4615]: https://github.com/JuliaLang/julia/issues/4615
@@ -495,6 +501,7 @@ 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
@@ -584,3 +591,4 @@ Deprecated or removed
 [#20543]: https://github.com/JuliaLang/julia/issues/20543
 [#20609]: https://github.com/JuliaLang/julia/issues/20609
 [#20889]: https://github.com/JuliaLang/julia/issues/20889
+[#21359]: https://github.com/JuliaLang/julia/issues/21359