Document top-level-division functionality with Docx (#10579)

Co-authored-by: John MacFarlane <jgm@berkeley.edu>

MANUAL.txt

@@ -1109,7 +1109,7 @@ header when requesting a document from a URL:
 `--top-level-division=default`|`section`|`chapter`|`part`
 
 :   Treat top-level headings as the given division type in
-    LaTeX, ConTeXt, DocBook, and  TEI output. The hierarchy
+    LaTeX, ConTeXt, DocBook, and TEI output. The hierarchy
     order is part, chapter, then section; all headings are
     shifted such that the top-level heading becomes the
     specified type. The default behavior is to determine the
@@ -1122,6 +1122,11 @@ header when requesting a document from a URL:
     headings to become `\part{..}`, while second-level headings
     remain as their default type.
 
+    In Docx output, this option adds section breaks before first-level
+    headings if `chapter` is selected, and before first- and second-level
+    headings if `part` is selected. Footnote numbers will restart
+    with each section break unless the reference doc modifies this.
+
 `-N`, `--number-sections=[true|false]`
 
 :   Number section headings in LaTeX, ConTeXt, HTML, Docx, ms, or EPUB