[css-rhythm-1] Add block-step-insert: content-box, and add -box suffixes. w3c#10486

Author: fantasai

css-rhythm-1/Overview.bs

@@ -207,8 +207,8 @@ Specifying the Spacing Type: the 'block-step-insert' property {#block-step-inser
 
 	<pre class="propdef">
 	Name: block-step-insert
-	Value: margin | padding
-	Initial: margin
+	Value: margin-box | padding-box | content-box
+	Initial: margin-box
 	Applies to: block-level boxes
 	Inherited: no
 	Percentages: N/A
@@ -218,21 +218,30 @@ Specifying the Spacing Type: the 'block-step-insert' property {#block-step-inser
 
 	This property specifies whether extra spacing
 	derived from applying 'block-step-size'
-	is inserted inside (like 'padding') or outside (like 'margin')
-	the box’s border.
+	is inserted as extra space inside (like 'padding')
+	or outside (like 'margin')
+	the box’s border,
+	or changes the height available to content
+	(like 'min-height' or 'max-height').
 
 	Values have the following meanings:
 
 	<dl dfn-for="block-step-insert" dfn-type="value">
-		<dt><dfn>margin</dfn>
+		<dt><dfn>margin-box</dfn>
 		<dd>
 			Any extra space resulting from a 'block-step-size'-induced adjustment
 			is inserted outside the box’s border, as extra margin.
 
-		<dt><dfn>padding</dfn>
+		<dt><dfn>padding-box</dfn>
 		<dd>
 			Any extra space resulting from a 'block-step-size'-induced adjustment
 			is inserted inside the box’s border, as extra padding.
+
+		<dt><dfn>content-box</dfn>
+		<dd>
+			Any extra space resulting from a 'block-step-size'-induced adjustment
+			is inserted inside the box’s border
+			by increasing the height of the [=content area=].
 	</dl>
 
 Specifying Alignment: the 'block-step-align' property {#block-step-align}