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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.56%. Comparing base (5771f3e) to head (cbf2275).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4010      +/-   ##
==========================================
- Coverage   79.58%   79.56%   -0.02%     
==========================================
  Files         141      141              
  Lines       15711    15711              
  Branches     2671     2671              
==========================================
- Hits        12504    12501       -3     
  Misses       2367     2367              
- Partials      840      843       +3     

Files with missing lines	Coverage Δ
sentry_sdk/tracing_utils.py	85.50% <ø> (ø)

... and 3 files with indirect coverage changes