[css-sizing-3] Use clamp() notation to define fit-content. w3c#4006

Author: fantasai

css-sizing-3/Overview.bs

@@ -244,7 +244,8 @@ Auto Box Sizes</h3>
 		<dt><dfn>fit-content block size</dfn>
 		<dd>
 			If the <a>available space</a> in a given axis is <a>definite</a>,
-			equal to <code>min(<a>max-content size</a>, max(<a>min-content size</a>, <a>stretch-fit size</a>))</code>.
+			equal to <code>clamp([=min-content size=], [=stretch-fit size=], [=max-content size=]
+			(i.e. <code>max(<a>min-content size</a>, min(<a>max-content size</a>, <a>stretch-fit size</a>))</code>).
 			Otherwise, equal to the <a>max-content size</a> in that axis.
 
 			Note: This is called the “shrink-to-fit” width in <a href="https://www.w3.org/TR/CSS2/visudet.html#float-width">CSS2.1§10.3.5</a>