Address async retry refactor review follow-ups

[jonathan343]

Overview

Minor follow-ups from the async RetryStrategy refactor (#671), addressing review feedback we deferred to a separate PR.

Changes

• Await the async transport send and deserialize calls in the designs/retries.md example.
• Import RetryStrategyType from ..retries in aio/retries.py instead of redefining the Literal alias.
• Fix the dangling RetryStrategy cross-reference in the RetryToken docstring, since the protocol moved to smithy_core.aio.interfaces.retries.
• Expand the changelog entry to note the module relocations, and add a trailing newline.

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