Add / fix smithy-client docs (#855)

* Add / fix smithy-client docs

* Rework hyper impls to be a regular public module instead of pub use

* clarify comments, fix changelog

* cleanup code in docs

* Add doc linking

* Update rust-runtime/aws-smithy-client/src/hyper_ext.rs

Co-authored-by: John DiSanti <jdisanti@amazon.com>

* fix changelog

* SDK changelog docs

Co-authored-by: John DiSanti <jdisanti@amazon.com>