Clarify Arize tracing integrations

[arizedatngo]

Summary

Clarifies the Arize entry in the OpenAI Agents SDK tracing ecosystem docs:

• adds a short Arize-specific note before the external processor list
• distinguishes Arize AX as the managed production tracing/evaluation path
• keeps Arize Phoenix as the open-source and self-hosted tracing path
• updates the displayed Phoenix name from Arize-Phoenix to Arize Phoenix
• links Arize's agent evaluation and LLM evaluation guides for teams using traces beyond debugging

This is a docs-only change.

Test plan

• git diff --check
• Manual docs review of the changed English sentences from Japanese, Korean, and Chinese translation perspectives; no ambiguous actor, scope, lifecycle boundary, or SDK identifier issue found.

I did not run the full repository verification stack because this PR only changes prose in docs/tracing.md.

Issue number

N/A - documentation clarification.

Checks

• I've added new tests, if relevant
• I've run .agents/skills/code-change-verification/scripts/run.sh
• I've confirmed all verification steps pass
• If using Codex, I've run /review before submitting this PR

[arizedatngo]

@william-xue @seratch I opened this as a docs-only cleanup to make the Arize tracing integration naming more precise and clarify the AX/Phoenix paths. You both have recent history on docs/tracing.md; could one of you take a look or route it to the right reviewer?

[seratch]

Thanks, we don't accept the additional text in the page but the service name will be updated by #4634