Rollup merge of #39065 - frewsxcv:libstd-os-unix-ffi-docs, r=GuillaumeGomez
Rollup merge of #39065 - frewsxcv:libstd-os-unix-ffi-docs, r=GuillaumeGomez

Add doc examples & description in `std::os::unix::ffi`.

None

graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust