Improve Mica/Acrylic descriptions on SystemBackdrops page

[niels9001]

Description

Note

This PR was generated with the assistance of GitHub Copilot (AI). All changes should be reviewed carefully.

Fixes #1869

Rewrites the SystemBackdrops page descriptions to clearly explain the difference between Mica and Acrylic backdrops.

Key improvements

• Lists all 4 backdrop kinds upfront: Mica, Mica Alt, Desktop Acrylic Base, Desktop Acrylic Thin
• Adds Mica vs. Acrylic comparison: Mica is opaque (wallpaper sampled once), Acrylic is semi-transparent (real-time blurred view)
• Removes misleading text: Old description said Acrylic is "used only for transient, light-dismiss surfaces" — this is true for in-app AcrylicBrush but not for DesktopAcrylicBackdrop window backdrops
• Fixes typo: "Inplemented" → clarified as separate AcrylicBrush concept
• Uses consistent terminology: "kinds" instead of "types" to match the API naming (MicaKind, DesktopAcrylicKind)
• Updates ControlInfoData.json description to match

Files changed

• SystemBackdropsPage.xaml — Rewritten all three description TextBlocks
• ControlInfoData.json — Updated the Description field for SystemBackdrops

Validation

1. Build: dotnet build WinUIGallery\WinUIGallery.csproj /p:Platform=x64 — 0 errors
2. Navigate to System Backdrops page and verify descriptions are readable and accurate