Document Tree documentation fixes.
Copy `topic` description text from the element docstring into the element
reference.
Describe a simplified "structure model" (that works with
`nodes.Element.check_content_model()`).
Simplify description of "%section.elements" parameter entity
(similar to the other "….elements" entities).
Remove an erroneous comment in the description of `transition`:
the additional placement constraints are indeed enforced in the
`misc.Transitions` transform.
Fix typos. Add/Fix links.
git-svn-id: http://svn.code.sf.net/p/docutils/code/trunk@9714 929543f6-e4f2-0310-98a6-ba3bd3dd1d04