Improve the RoundingMode case descriptions with regard to rounding to a specified number of decimals

> The bit with the decimal part might need some adjustments either in the documentation for `round()` or in here, because `round()` supports rounding to a specified precision instead of integers.

_Originally posted by @TimWolla in https://github.com/php/doc-en/pull/4080#discussion_r1845547037_
            