Clarify UIElement.RasterizationScale semantics and XamlRoot relationship

[Copilot]

UIElement.RasterizationScale did not explain its intended use, which made it easy to misread as the current display scale. This update clarifies that it is an element-level rendering override and distinguishes it from XamlRoot.RasterizationScale.

• Documentation clarity

  • Updated the property description to make it explicit that UIElement.RasterizationScale is an extra scale factor you set for rendering an element's content.
  • Removed ambiguity that could imply the property reports monitor or view scale.

• Behavior and defaults

  • Documented the primary use case: rendering an element at higher resolution than its layout size, such as before scale animation.
  • Explained why the default is 1.0: no extra per-element rasterization scaling is applied unless requested.

• Relationship to XamlRoot.RasterizationScale

  • Added explicit guidance that XamlRoot.RasterizationScale is the read-only source for the XAML tree's current view-to-physical-pixel scale.
  • Clarified that UIElement.RasterizationScale is an additional per-element multiplier layered on top of that environment scale.

Example of the distinction:

// Read the display scale for the current XAML tree.
double displayScale = myElement.XamlRoot.RasterizationScale;

// Optionally apply extra rasterization scale for a specific element.
myElement.RasterizationScale = 2.0;

• Fixes No information about the purpose of UIElement.RasterizationScale #95