[css-sizing-4] Add cross-references.

Author: fantasai

css-sizing-4/Overview.bs

@@ -108,6 +108,7 @@ New Sizing Values: the ''stretch'', ''fit-content'', and ''contain'' keywords</h
 		<dd>
 			Applies [=stretch-fit sizing=],
 			attempting to match the size of the box’s [=margin box=] to the size of its [=containing block=].
+			See [[#stretch-fit-sizing]].
 
 		<dt><dfn>fit-content</dfn>
 		<dd>
@@ -119,6 +120,7 @@ New Sizing Values: the ''stretch'', ''fit-content'', and ''contain'' keywords</h
 			applies [=contain-fit sizing=],
 			attempting to fit into the box’s constraints
 			while maintaining its [=preferred aspect ratio=] insofar as possible.
+			See [[contain-fit-sizing]].
 
 			If the box has no [=preferred aspect ratio=],
 			applies [=stretch-fit sizing=].