Add missing documentation.

[AureliaDolo]

Fix #153

Hello, here are some bits of doc, and the the warnings.

Thanks!

[AureliaDolo]

The remaining clippy warnings are:

• safety doc for RwRevIter::append
• safety doc for Env::copy_to_fd
• module inception for iter

I'd happily take your suggestions on these !

[Kerollmops]

I merge this PR as it looks good to me and I am awaiting the changes in #178 🎉