[css-sizing-3] Add note to clarify indefiniteness of intrinsic size keywords

See #7206 (comment)

Author: frivoal

css-sizing-3/Overview.bs

@@ -148,6 +148,10 @@ Terminology</h2>
 			A size that is not <a>definite</a>.
 			<a>Indefinite</a> <a>available space</a> is essentially infinite.
 
+			Note: [=intrinsic sizing=] keywords such as ''width/max-content'' are indefinite,
+			even if they can be determined without laying out the children
+			e.g. due to [=size containment=] or lack of children.
+
 		<dt><dfn id="available" local-lt="available" lt="available space|available inline space|available block space">available space</dfn>
 		<dd>
 			A size representing the space into which a box is laid out,