Wrong heading sizes for sections included via toctrees

[teake]

The heading size for sections included via toctrees is rendered incorrectly in Word. Consider this example:

• index.rst:

  *************
  Chapter title
  *************
  
  .. toctree::
  
     section

• section.rst:

  Section title
  =============
  
  .. toctree::
  
     subsection

• subsection.rst:

  Subsection title
  ----------------
  
  Some text goes here.

In the resulting Word file, all headings are of the same size (namely Heading 1). This is incorrect -- they should have descending header sizes.

When compiling the above example to singlehtml, the resulting HTML file does have the correct descending header sizes (namely <h1>, <h2>, and <h3>).

In contrast, when putting all the content in a single .rst file:

*************
Chapter title
*************

Section title
=============

Subsection title
----------------

Some text goes here.

the resulting Word file does have the correct header sizes.