[css-inline] add illustration showing how initial leter placement is not affected by the content of spanned lines, per w3c#735

Author: dauwhe

css-inline-3/Overview.bs

@@ -637,6 +637,12 @@ Layout Model</h4>
 			the <a>initial letter</a> itself
 			followed by an infinite sequence of plain text
 			as the direct contents of its <a>root inline box</a>.
+			
+			<figure>
+				<img src="images/infinite-text.png" width="300"
+		     alt="text underlay shows how initial letter alignment is not affected
+		     by the content of the spanned lines">
+			</figure>
 
 		<li>
 			Exclude content within the inline letter's margin box