Clarified documentation on the AllowEditInvariantFromNonDefault setting

[AndyButland]

📋 Description

Having investigated a reported issue on the CMS, it was clear our documentation of this setting wasn't entirely correct and was missing some changes between versions.

I've corrected the details of the default value of the setting across all versions, and updated the copy for 15 and 16 to describe the current behaviour.

📎 Related Issues (if applicable)

umbraco/Umbraco-CMS#19814

✅ 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 10+

Deadline (if relevant)

Any time