Add details of Examine customization breaking change in Umbraco 16

[AndyButland]

📋 Description

Add details of Examine set up amend Umbraco 16 that causes a breaking change for search customisation code.

📎 Related Issues (if applicable)

umbraco/Umbraco-CMS#20267

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Relevant pages are linked.
• All links work and point to the correct resources.
• Screenshots or diagrams are included if useful.
• Any code examples or instructions have been tested.
• Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

Umbraco 16+

Deadline (if relevant)

Any time.

[eshanrnh]

Thanks for the PR, @AndyButland 🙌 The PR looks good to me,