[css-fonts-4] font-optical-sizing interacts with font-size-adjust

Fixes w3c#6190.

Author: litherum

css-fonts-4/Overview.bs

@@ -5587,6 +5587,9 @@ User agents must not select a value for the "opsz" axis
 which is not supported by the font used for rendering the text.
 This can be accomplished by clamping a chosen value to the range supported by the font.
 
+'font-optical-sizing' interacts with 'font-size-adjust'. When applying optical sizing, User agents must
+apply an optical sizing value appropriate for the adjusted font size, subject to the above restrictions.
+
 <h3 id="font-variation-settings-def">
 Low-level font variation settings control: the 'font-variation-settings' property</h3>