add missing doc lint and and missing docs to all runtime crates

[rcoh]

Add the missing_docs and missing_crate_level_docs lints to all the runtime crates. For example:

#![warn(
    missing_docs,
    rustdoc::missing_crate_level_docs,
)]

AWS runtime crates:

• aws-config
• aws-endpoint (deprecated)
• aws-http
• aws-hyper (deprecated)
• aws-inlineable
• aws-sig-auth (deprecated)
• aws-sigv4
• aws-types
• aws-credential-types
• aws-runtime
• aws-runtime-api

Smithy runtime crates:

• aws-smithy-async
• aws-smithy-checksums
• aws-smithy-client (deprecated)
• aws-smithy-eventstream
• aws-smithy-http
• aws-smithy-http-tower (deprecated)
• aws-smithy-json
• aws-smithy-protocol-test
• aws-smithy-query
• aws-smithy-runtime
• aws-smithy-runtime-api
• aws-smithy-types
• aws-smithy-types-convert
• aws-smithy-wasm
• aws-smithy-xml