docs: Added xml comment, and placed the namespace to correct position…

[VaclavElias]

PR Details

XML improvements, GraphicsCompositor class improved with more content and a link to our manual.

Related Issue

Fixes #2306

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have built and run the editor to try this change out.

[Eideren]

Thanks !