docs: align retry and pipeline docs

[WilliamK112]

Summary

• add operation-tracing to the README default_pipeline canonical policy stack
• reword docs/bodies.md so retry body buffering is keyed to the effective per-call retry total
• align the body buffering docs with docs/pipelines.md and RetryPolicy.send

Closes #47
Closes #48

Validation

• uv sync --dev
• fixed-string checks confirmed README includes operation-tracing and docs/bodies.md no longer uses constructor-only total_retries > 0 / total_retries == 0 wording
• git diff --check
• uv run ruff check .
• uv run mypy
• uv run pytest, 1412 passed