[motion-1] Clarify meaning of contain

'smallest clipped area' gave no guidance when the path and the box
had zero area in common regarless of the value of offset-distance.

For example, this can occur when the path size is 0, or when
offset-anchor is substantially outside the box.

We now define the meaning of 'contain' by specifying that the path
size is minimally increased so that the box can be contained.

resolves w3c#22

motion-1/Overview.bs

@@ -182,9 +182,9 @@ Values have the following meanings:
 <dl>
 		<dt><dfn>contain</dfn></dt>
 		<dd>
-			Makes the box don't have clipped area by altering the length of the <a>offset path</a> when the part of the box on the <a>offset path</a> is outside the edge of the containing block.
-			If the box is larger than the containing block, the box would be positioned where it has the smallest clipped area by modifying the <a>offset path</a>’s length with reducing the least amount.
+			The used value of 'offset-distance' is clamped so that the box is entirely contained within the path.
 
+			If no 'offset-distance' would lead to the box being enclosed by the path, the path size is minimally increased so that such an 'offset-distance' exists.
 		</dd>
 </dl>
 
@@ -258,6 +258,35 @@ Values have the following meanings:
 			<img alt="An image of boxes positioned with contain" src="images/offset_distance_with_contain.png" style="width: 200px;"/>
 			<figcaption>'offset-path' with 'contain'</figcaption>
 		</div>
+		In the third example, the path size is increased so that the box can be contained. The <a>used offset distance</a> is negative.
+		<pre class="lang-html">
+			&lt;style>
+			  body {
+			    transform-style: preserve-3d;
+			    width: 250px;
+			    height: 250px;
+			  }
+			  .box {
+					width: 60%;
+					height: 10%;
+
+					offset-position: 20% 20%;
+					offset-distance: 0%;
+					offset-anchor: 200% -300%;
+			  }
+			  #blueBox {
+			    background-color: blue;
+			    offset-path: ray(-90deg closest-side contain);
+			  }
+			&lt;/style>
+			&lt;body>
+			  &lt;div class="box" id="blueBox">&lt;/div>
+			&lt;/body>
+		</pre>
+		<div class=figure>
+			<img alt="An image of an increased path size" src="images/increase-size.svg" style="width: 400px; height: 335;"/>
+			<figcaption>'offset-path' with path size increased</figcaption>
+		</div>
 		</div>
 </dd>
 
@@ -431,7 +460,7 @@ To determine the <dfn>used offset distance</dfn> for a given <a>offset path</a>
     :   If <a>offset path</a> is an unbounded ray:
     ::  Let <a>used offset distance</a> be equal to <a>offset distance</a>.
     :   Otherwise if <a>offset path</a> is an <<angle>> path with contain:
-    ::  Let <a>used offset distance</a> be equal to <a>offset distance</a>, clamped in magnitude by the total length of the path.
+    ::  Let <a>used offset distance</a> be equal to <a>offset distance</a>, clamped so that the box lies entirely within the path.
     :   If <a>offset path</a> is any other unclosed interval:
     ::  Let <a>used offset distance</a> be equal to <a>offset distance</a> clamped by 0 and the total length of the path.
     :   Otherwise <a>offset path</a> is a closed loop: