[css-sizing-3] Add inner-size and outer-size images. w3c#1938

Author: tabatkins

css-sizing-3/Overview.bs

@@ -114,10 +114,20 @@ Terminology</h2>
 		<dd>
 			The <a href="https://www.w3.org/TR/css2/box.html#box-dimensions">content-box</a> [=size=] of a <a>box</a>.
 
+			<figure>
+				<img src="images/inner-size.svg" alt="">
+				<figcaption>Inner size</figcaption>
+			</figure>
+
 		<dt><dfn lt="outer size|outer width|outer height|outer block size|outer inline size">outer size</dfn>
 		<dd>
 			The <a href="https://www.w3.org/TR/css2/box.html#box-dimensions">margin-box</a> [=size=] of a <a>box</a>.
 
+			<figure>
+				<img src="images/outer-size.svg" alt="">
+				<figcaption>Outer size</figcaption>
+			</figure>
+
 		<dt><dfn id="definite" lt="definite|definite size">definite size</dfn>
 		<dd>
 			A size that can be determined without performing layout;