docs: clarify baggage current context behavior

[goutamadwant]

Description

Clarifies that baggage helpers return updated Context values and do not attach those values as the current context automatically.

The docs now show the difference between reading baggage from an explicit context and reading baggage from the current context. They also include the context.attach / context.detach pattern needed when callers want a returned context to become current.

Fixes #2432

Type of change

• This change requires a documentation update

How Has This Been Tested?

• uv run tox -e docs
• Ran the documented baggage/context snippets locally

Does This PR Require a Contrib Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests are not applicable for this documentation-only change
• Documentation has been updated

[carlosalberto]

Actually it's the other way around: baggage follows what is expected to do by the Context API.

[carlosalberto]

i.e. no need to mention Baggage here.