Rollup merge of #103236 - tspiteri:redoc-int-adc-sbb, r=m-ou-se

doc: rewrite doc for signed int::{carrying_add,borrowing_sub}

Reword the documentation for bigint helper methods, signed `int::{carrying_add,borrowing_sub}` (#85532).

This change is a follow-up to #101889, which was for the unsigned methods.

Author: compiler-errors