Some limitations of the brief sample Jon provided and IIRC of DocBook
(but lord knows, I am no DocBook expert):
* Only provides for 1 index.
* Definitionally limited number of subcatagories (this might be a
good thing, though :-)
* Cannot declare the language of the index term as being different
from the language of the content.
TEI indexing takes care of all three of these concerns. See
http://www.tei-c.org/release/doc/tei-p5-doc/html/CO.html#CONOIX.