GH-22342: [C++] [Documentation] Clarify Union type ids

[erichanwang]

Rationale for this change

The Union layout documentation does not explain that the type IDs stored in the types buffer can differ from the child array indices, or how the optional typeIds metadata maps children to physical type IDs.

What changes are included in this PR?

• Document the relationship between Union child array indices, physical type IDs, and the optional typeIds metadata.
• Clarify that the types buffer stores the type ID for each slot.

Fixes #22342.

Are these changes tested?

git diff --check passes and the RST change was reviewed against Schema.fbs and the existing Union layout documentation. A local Sphinx/RST checker is not installed in this environment.

Are there any user-facing changes?

No.

• GitHub Issue: [C++] [Documentation] add discussion of Union.typeIds to Layout.rst  #22342

[github-actions]

⚠️ GitHub issue #22342 has been automatically assigned in GitHub to PR creator.

[erichanwang]

Formatting follow-up:

• Wrapped the new Union type-id paragraph to match the surrounding Arrow RST line-wrapping style.
• git diff --check passes; no semantic content changed.

[uros-b]

Nice focused improvement, LGTM!