docs(baggage): Document that caller must check mutable (#4010)

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

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Co-authored-by: Anton Pirker <anton.pirker@sentry.io>

Author: szokeasaurusrex

sentry_sdk/tracing_utils.py

@@ -543,6 +543,10 @@ def _sample_rand(self):
 class Baggage:
     """
     The W3C Baggage header information (see https://www.w3.org/TR/baggage/).
+
+    Before mutating a `Baggage` object, calling code must check that `mutable` is `True`.
+    Mutating a `Baggage` object that has `mutable` set to `False` is not allowed, but
+    it is the caller's responsibility to enforce this restriction.
     """
 
     __slots__ = ("sentry_items", "third_party_items", "mutable")