docs(baggage): Document that caller must check mutable

[szokeasaurusrex]

The Baggage class does not enforce mutability. Document this to avoid confusion.

---

Thank you for contributing to sentry-python! Please add tests to validate your changes, and lint your code using tox -e linters.

Running the test suite on your PR might require maintainer approval. The AWS Lambda tests additionally require a maintainer to add a special label, and they will fail until this label is added.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
22038	1	22037	5702

View the top 1 failed tests by shortest run time

tests.integrations.aiohttp.test_aiohttp test_span_origin[pyloop]

Stack Traces | 34s run time

.../integrations/aiohttp/test_aiohttp.py:612: in test_span_origin
    (event,) = events
E   ValueError: too many values to unpack (expected 1)

To view more test analytics, go to the Test Analytics Dashboard
📢 Thoughts on this report? Let us know!