docs(requests): clarify hook mutation and propagation order

[jstar0]

Summary

Fixes #4990.

Clarify that request_hook receives the live requests.PreparedRequest, that mutations affect the outbound request, and that propagation injection runs afterward and may replace propagation headers. Add the same guidance to the package README and API documentation, including a small header-mutation example.

Testing

• env -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u http_proxy -u https_proxy -u all_proxy tox -e py312-test-instrumentation-requests (80 passed)
• Ruff check and format check for the changed Python module
• Sphinx requests page build succeeded with optional database imports mocked; the full docs build is blocked on the host missing libmysqlclient.24.dylib while importing the unchanged MySQL instrumentation
• git diff --check

Documentation-only change; no core repository change or changelog fragment is required.

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Waiting on reviewers · refreshed 2026-08-29 12:26 UTC

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