docs: clarify baggage and current context semantics

[iacker]

Summary

• explain that baggage helpers return new Context snapshots instead of mutating the current context
• document the difference between explicit context=... parameters and the current execution context
• show the correct context.attach/context.detach pattern for baggage

Test Plan

• tox -e docs
  • the docs build is currently blocked in this environment by an external intersphinx fetch failure (pymongo.readthedocs.io inventory resolution), which Sphinx treats as an error here
  • the content change itself is limited to docs/api/baggage.rst and docs/api/context.rst

Refs #2432

[linux-foundation-easycla]

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• ✅ login: iacker / name: Billard (9de347c)