add NEWS.md entry for three-argument rem and rem2pi (#21812)

* Add NEWS.md entry for three-argument rem and the new rem2pi (#10946).

* Clarify precise behaviour

NEWS.md

@@ -414,6 +414,12 @@ Library improvements
     Currently, it is used by default for the new `RowVector` type only, and
     enforces that both `transpose(vec)` and `ctranspose(vec)` are views not copies ([#20047]).
 
+  * `rem` now accepts a `RoundingMode` argument via `rem(x, y, r::RoundingMode)`, yielding
+    `x - y*round(x/y, r)` without intermediate rounding. In particular, `rem(x, y, RoundNearest)`
+    yields a value in the interval `[-abs(y)/2, abs(y)/2]`), which corresponds to the IEE754
+    `remainder` function. Similarly, `rem2pi(x, r::RoundingMode)` now exists as well, yielding
+    `rem(x, 2pi, r::RoundingMode)` but with greater accuracy ([#10946]).
+
   * `map[!]` and `broadcast[!]` now have dedicated methods for sparse/structured
     vectors/matrices. Specifically, `map[!]` and `broadcast[!]` over combinations including
     one or more `SparseVector`, `SparseMatrixCSC`, `Diagonal`, `Bidiagonal`, `Tridiagonal`,
@@ -574,6 +580,7 @@ Command-line option changes
 [#7669]: https://github.com/JuliaLang/julia/issues/7669
 [#8974]: https://github.com/JuliaLang/julia/issues/8974
 [#9343]: https://github.com/JuliaLang/julia/issues/9343
+[#10946]: https://github.com/JuliaLang/julia/issues/10946
 [#11250]: https://github.com/JuliaLang/julia/issues/11250
 [#11310]: https://github.com/JuliaLang/julia/issues/11310
 [#12274]: https://github.com/JuliaLang/julia/issues/12274