Rollup merge of #42091 - maccoda:maccoda/env_docs, r=frewsxcv

Rollup merge of #42091 - maccoda:maccoda/env_docs, r=frewsxcv

Improving std::env docs

Addresses #29351.
Hopefully this addresses the following points:
> -  iterators should use the standard iterator boilerplate like https://doc.rust-lang.org/std/iter/struct.Map.html, this applies to all structs except for JoinPathsError
> -  JoinPathsError should properly link the function it comes from and use language similar to https://doc.rust-lang.org/std/io/struct.Error.html
> -  same wording issues with VarError
> - functions need to ensure linkage to things they refer to in their descriptions
> - Explain the difference between `os` and non-`os` structs and methods