Fix mismatch between Docutils DTD and actual behaviour.
The rST parser sets the "xml:space" attribute on the `<math_block>`
doctree element since support this element in Docutils 0.8.
The documentation of the new element in the DTD missed this fact.
Add "xml:space" to the !ATTLIST so that "Docutils XML" documents
containing math blocks validate.
The `<raw>` element may only contain text, no inline elements.
git-svn-id: http://svn.code.sf.net/p/docutils/code/trunk@9546 929543f6-e4f2-0310-98a6-ba3bd3dd1d04