Update remarks (dotnet#5317)

Use the (much better) actual GDI docs https://docs.microsoft.com/en-us/windows/win32/api/gdiplusenums/ne-gdiplusenums-compositingquality?redirectedfrom=MSDN

Author: john-h-k

xml/System.Drawing.Drawing2D/CompositingQuality.xml

@@ -30,8 +30,9 @@
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 ## Remarks  
- Compositing is done during rendering when the source pixels are combined with the destination pixels to produce the resultant pixels. The quality of compositing directly relates to the visual quality of the output and is inversely proportional to the render time. The higher the quality, the slower the render time. This is because the higher the quality level, the more surrounding pixels need to be taken into account during the composite. The linear quality setting (AssumeLinear) compromises by providing better quality than the default quality at a slightly lower speed.  
-  
+When you specify that gamma correction should not be applied, the image data to be rendered (blended with the background) is assumed to be in a linear color space with a gamma value of 1.0. As a result, no gamma adjustment is applied to the image data before or after blending the image with the background.
+
+When you specify that gamma correction should be applied, the image data to be rendered (blended with the background) is assumed to be in the sRGB color space with a gamma value of 2.2. To ensure accurate blending, the input image data is transformed into a linear (gamma = 1.0) space before the colors are blended and transformed back into sRGB (gamma = 2.2) space afterward. This mode results in a more accurate blend at the expense of additional processing time.
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