Rollup merge of #124701 - scottmcm:unchecked_sub_docs, r=Nilstrieb

Docs: suggest `uN::checked_sub` instead of check-then-unchecked

As of #124114 it's exactly the same in codegen, so might as well not use `unsafe`.

Note that this is only for *unsigned*, since the overflow conditions for `iN::checked_sub` are more complicated.

Author: matthiaskrgr