Indicate expected improvement in BcMath's performance

[FleetingCloud]

Description

Hi, first of all, thanks a lot for all the work done on BcMath especially Saki Takamachi, nielsdos and Gina.
Currently NEWS file only include following about it:

Improve performance. (Saki Takamachi, nielsdos)

I think it should also tell by how much . It's because a person will likely think that improvement is modest if they just read above news entry. In reality, there is a great performance boost.
By including some benchmark, it will encourage people heavily using BcMath to update their PHP version sooner.
It will also help improve perception of "BCMath is slow" which it is not now.
Also spread more awareness.
Again thank a lot.

Cc @SakiTakamachi

[Girgias]

This seems more like a documentation issue for the PHP 8.4 migration guide.

The same could also be said for the DOM extension, and others.

[SakiTakamachi]

Ah, sorry. There are so many refactorings, I was thinking of providing more detailed information after everything is finished, if necessary.

As you mentioned there could probably be a bit more detail, so I'll update this once the refactoring is settled.

[FleetingCloud]

No problem.
Take your time. Thanks again.