Rollup merge of rust-lang#146925 - DiuDiu777:va-doc-fix, r=tgross35

Add doc for va_list APIs

I observed that [PR146521](rust-lang#146521) submitted two weeks ago resolved some documentation issues related to `VaListImpl`, similar to the previous [PR136969](rust-lang#136969).

This PR specifically adds requirements about argument availability for `VaListImpl::arg`, and also adds safety descriptions to the three associated intrinsic APIs.

Author: matthiaskrgr