Reconcile Docutils DTD and Document Tree documentation.

Reconcile Docutils DTD and Document Tree documentation.

There are some cases where docutils.dtd and doctree.txt diverged.
Change so, that the documentation is consistent and matches actual behaviour.

Remove declaration of element `<info>`:

* added to docutils.dtd and nodes.py but not doctree.txt 2004-03-21,
* never used,
* removed from nodes.py 2004-11-09.

Remove `<decoration>` from content model of `<section>` elements:

* added to docutils.dtd but not doctree.txt 2004-03-21,
* never used because the Docutils rST parser inserts `<decoration>`
  directly into `<document>` even if the "header" or "footer" directive
  is nested in a section.

Fix declaration of `<sidebar>` content:

* no `<subtitle>` without `<title>`.

Add `<subtitle>` to children of `<section>` elements in "doctree.txt":

* present in docutils.dtd,
* used in the functional test examples via SectSubTitle transform
  (with "sectsubtitle_xform" configuration setting True).

git-svn-id: http://svn.code.sf.net/p/docutils/code/trunk@9647 929543f6-e4f2-0310-98a6-ba3bd3dd1d04