clarify breakpoints as being between *sibling* block boxes

Author: fantasai

css3-page/Overview.html

@@ -2503,9 +2503,9 @@ <h3 id=allowed-pg-brk><span class=secno>5.4 </span>Allowed page breaks</h3>
      the following places:</p>
 
     <ol>
-     <li>In the vertical margin between block boxes (or rows in a table).
-      When a page break occurs here, the used values of the adjoining <span
-      class=property>'margin-bottom'</span> and <span
+     <li>In the vertical margin between sibling block boxes (or rows in a
+      table). When a page break occurs here, the used values of the adjoining
+      <span class=property>'margin-bottom'</span> and <span
       class=property>'margin-top'</span> properties are set to <span
       class=css>'0'</span>.
 

css3-page/Overview.src.html

@@ -1868,7 +1868,7 @@ <h3 id="allowed-pg-brk">Allowed page breaks</h3>
 following places:</p>
 
 <ol>
-<li>In the vertical margin between block boxes (or rows in a table).
+<li>In the vertical margin between sibling block boxes (or rows in a table).
 When a page break occurs here, the used values of the adjoining <span class="property">'margin-bottom'</span> and <span class="property">'margin-top'</span> properties are set to <span class="css">'0'</span>.</li>
 <li>Between line boxes inside a block box.</li>
 </ol>