improve ByteStream docs & add pub use (#443)

* improve ByteStream docs & add pub use

This commit attempts to improve the discoverability and useability of `ByteStream`:
1. Adds pub use for bytestream. This is currently added to all crates. At some point in the future, we may wish to prune this
to crates that actually use it, but as it doesn't actually add an additional dependency, there isn't currently any harm.
SdkBody is explicitly _not_ pub use'd. Generally, people shouldn't need to use `SdkBody` directly.

2. Clarifies, adds examples, and generally increases the scope of the `ByteStream` documentation.

* Update rust-runtime/smithy-http/src/byte_stream.rs