API Docs: env

[steveklabnik]

Part of #29329

http://doc.rust-lang.org/std/env/

• 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

[GuillaumeGomez]

Seems the docs are all good here.

[steveklabnik]

There's still a bunch to do here; the module documentation is very sparse, not everything has examples, not all types are linked correctly.

[GuillaumeGomez]

I'll try to look for the missing ones then.

[steveklabnik]

I am happy to mentor anyone who wants to tackle this issue.

[mp4096]

I'd like to work on this one. Here are some ideas:

• Hoist some examples (e.g. using env vars or command line args) to the module-level documentation
• Explain the difference between os and non-os structs and methods
• Add explanation text for example snippets

@steveklabnik Maybe you have more ideas for a checklist? Thanks!