Missing NEWS entry for the use of `ScopedValue` in `setprecision`

[Joel-Dahne]

The change made in #51362 to make setprecision use ScopedValue does not seem to be included in the NEWS.md file. It seems like a valuable change to mention since it now allows using setprecision in a threaded save way.

I also noticed that the docstring for setprecision has not been updated to mention anything about ScopedValue.

[JeffBezanson]

Fixed and will be backported by #58699