std: Improve some I/O documentation

This lowers the #[allow(missing_doc)] directive into some of the lower modules
which are less mature. Most I/O modules now require comprehensive documentation.
auto merge of #12607 : alexcrichton/rust/io++, r=brson

This lowers the #[allow(missing_doc)] directive into some of the lower modules
which are less mature. Most I/O modules now require comprehensive documentation.