Enable validation of Docutils XML documents against the DTD:
Use attribute type NMTOKEN instead of REFID for the `refid` attribute
and NMTOKENS for `backrefs`: REFID refers to an ID type instance,
however, the `ids` attribute cannot use the ID type because `XML only
allows one ID per Element Type`__ and does not support a multiple-ID
"IDS" attribute type.
__ https://www.w3.org/TR/REC-xml/#sec-attribute-types
git-svn-id: http://svn.code.sf.net/p/docutils/code/trunk@8031 929543f6-e4f2-0310-98a6-ba3bd3dd1d04