Clarify some manifest values and add examples

[sredna]

📖 Description

• Tries to clarify the description of some of the manifest values.
• Adds value examples.
• Adds the UpgradeCode definition.

Notes:

• UpgradeCode is restricted only to GUIDs because this is a MSI feature. I have verified that all existing manifests in the community repository only uses GUIDs here.
• DefaultInstallLocation is restricted to strings starting with % or \. The thinking is that the only sane values are %ProgramFiles%\MyApp, %LOCALAPPDATA%\Programs\MyApp and \AppInRootOfDrive.

🔗 References

Fixes some of the issues reported in #6221

🔍 Validation

None/YOLO

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated Release Notes (if applicable)
• Updated documentation (if applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)

📋 Issue Type

• Bug fix
• Feature
• Task

Microsoft Reviewers: Open in CodeFlow