Added details of email expiry configuration

[AndyButland]

📋 Description

Added configuration details for email expiry.

📎 Related Issues (if applicable)

umbraco/Umbraco-CMS#20285

✅ 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)

CMS 16.4 and 17.1

Deadline

Should be published to coincide with the 16.4 release candidate, which is scheduled for 13th November.

[eshanrnh]

Thanks for the PR, @AndyButland 🙌 The PR looks great.

Will merge it on the 13th of November.