[css-fonts-4] add warning on too large x-height

Author: svgeesus

css-fonts-4/Overview.bs

@@ -1008,6 +1008,10 @@ Relative sizing: the 'font-size-adjust' property</h3>
 	and is equal to the x-height of a font
 	divided by the font size.
 
+	Note: For text which uses diacritics,
+	too large an x-height will actually decrease legibility
+	as the diacritics become cramped.
+
 	In situations where font fallback occurs,
 	fallback fonts might not share the same aspect value
 	as the desired font family