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).