Modify str Structs descriptions

References #29375. Modified descriptions of multiple structs to be more in line with structs found under [`std::iter`](https://doc.rust-lang.org/std/iter/#structs), such as [`Chain`](https://doc.rust-lang.org/std/iter/struct.Chain.html) and [`Enumerate`](https://doc.rust-lang.org/std/iter/struct.Enumerate.html)