Update php.md

[elee0625]

What does this PR do?

PHP Doc Simplification

Motivation

Onboarding Initiative

Additional Notes

N/A

---

Reviewer checklist

• [ ] Review the changed files.
• [ ] Review the URLs listed in the Preview section.
• [ ] Check images for PII
• [ ] Review any mentions of "Contact Datadog support" for internal support documentation.

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/eddy/php-public-docs/tracing/trace_collection/dd_libraries/php