Docx writer: restart footnotes by section by default.

This can be overridden by a final sectPr element in the body
of the reference.docx.

It will only change things for `--top-level-division=chapter`,
since only top-level chapters are put in separate sections.
For that use it will mean that footnote numbers start over with
each chapter, which is usually what is wanted.

Closes #2773.

data/docx/word/document.xml

@@ -189,7 +189,7 @@ Body Text. Body Text Char.
       </w:r>
       <w:r>
         <w:t xml:space="preserve">
- 
+
 </w:t>
       </w:r>
       <w:r>
@@ -207,7 +207,7 @@ Verbatim Char
       </w:r>
       <w:r>
         <w:t xml:space="preserve">
- 
+
 </w:t>
       </w:r>
       <w:hyperlink r:id="rId30">
@@ -227,7 +227,7 @@ Hyperlink
       </w:r>
       <w:r>
         <w:t xml:space="preserve">
- 
+
 </w:t>
       </w:r>
       <w:r>
@@ -391,6 +391,10 @@ Definition
 </w:t>
       </w:r>
     </w:p>
-    <w:sectPr />
+    <w:sectPr>
+      <w:footnotePr>
+        <w:numRestart w:val="eachSect" />
+      </w:footnotePr>
+    </w:sectPr>
   </w:body>
 </w:document>

data/templates/default.openxml

@@ -64,7 +64,11 @@ $-- sectpr will be set to the last sectpr in a reference.docx, if present
 $if(sectpr)$
     $sectpr$
 $else$
-    <w:sectPr />
+    <w:sectPr>
+      <w:footnotePr>
+        <w:numRestart w:val="eachSect" />
+      </w:footnotePr>
+    </w:sectPr>
 $endif$
   </w:body>
 </w:document>