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
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