[css-sizing-3] Actually define the term "intrinsic sizes"

Closes #3680

Author: frivoal

css-sizing-3/Overview.bs

@@ -782,6 +782,9 @@ Intrinsic Sizes</h3>
 	and if its containing block was <em>infinitely</em>-sized in that axis.
 	(In other words, the maximum size it has when sized as “shrink-to-fit”.)
 
+	The [=min-content size=] and [=max-content size=] are collectively referred to
+	as the <dfn export id=dfn-intrinsic-sizes>intrinsic sizes</dfn>.
+
 	Note: Note that when the box has an intrinsic aspect ratio,
 	size constraints in the opposite dimension will transfer through
 	and can affect the ''width/auto'' size in the considered one.