[css-rhythm-1] Handle fragmentation in general, not just page breaks. Adjusts wording added in #1260

Author: fantasai

css-rhythm-1/Overview.bs

@@ -25,6 +25,7 @@ spec:css21; type:property; text:max-width
 spec:css21; type:property; text:min-width
 spec:css-writing-modes-4; type:dfn; text:end
 spec:css-writing-modes-4; type:dfn; text:start
+spec:css-break-3; type:dfn; text:fragment
 </pre>
 
 Introduction {#intro}
@@ -182,10 +183,13 @@ Specifying the Step Size: the 'block-step-size' property {#block-step-size}
 	Values other than ''block-step-size/none''
 	cause the box to establish a new formatting context.
 
-	In situations where margins <a href="">collapse</a>,
+	In situations where margins <a href="https://www.w3.org/TR/CSS2/box.html#collapsing-margins">collapse</a>,
 	only the box’s own margin is considered
 	in calculating its outer size.
 
+	When a box <a>fragments</a>,
+	step sizing is applied per fragment.
+
 Specifying the Spacing Type: the 'block-step-insert' property {#block-step-insert}
 --------------------------------------------------------
 
@@ -264,11 +268,11 @@ Specifying Alignment: the 'block-step-align' property {#block-step-align}
 
 	In all cases,
 	additional spacing cannot be added to margins
-	that are truncated due to unforced page breaks
-	(see [[CSS3-PAGE]]);
+	that are truncated or omitted due to unforced fragmentation breaks
+	(see [[CSS3-PAGE]] and [[CSS-BREAK-3]]);
 	therefore if 'block-step-insert' is ''margin'',
 	all extra space derived from applying 'block-step-size'
-	must be inserted on the opposite side of the box
+	must be inserted on the opposite side of the fragment
 	(regardless of 'block-step-align').
 
 Rounding Method: the 'block-step-round' property {#block-step-round}