add documentation for resolution scale

docs/source/reference/modding/settings/video.rst

@@ -23,6 +23,23 @@ Video Settings
    Larger values produce more detailed images within the constraints of your graphics hardware,
    but may reduce the frame rate.
 
+.. omw-setting::
+   :title: resolution scale
+   :type: float
+   :range: 0.1 to 1.0
+   :default: 1.0
+   :location: :bdg-info:`In Game > Options > Video`
+
+   This setting controls the internal rendering resolution as a percentage of the window resolution.
+   Values below 1.0 render the game at a lower resolution and upscale to the window size.
+   For example, 0.5 on a 2560x1440 display would render at 1280x720 internally,
+   providing substantial performance improvements on GPU-limited systems while maintaining UI elements at full resolution.
+   
+   .. note::
+      This setting is primarily intended for low-powered devices with high-DPI displays
+      (such as retro handhelds or low-powered Android phones). On desktop systems with adequate GPUs, 
+      this should typically remain at 1.0 for optimal image quality.
+
 .. omw-setting::
    :title: window mode
    :type: int