Replace revision and publication date in generated documentation with
the actual date of last update. This fixes the problems with misleading
"last update" dates that only reflect the change in a single
rarely-changed file. Note that hrs had reservations about doing this in
the Makefile rather than with a stylesheet. However, not all our source
files are XML. For now, this change works, and can be replaced by a
more elegant solution later. We should also consider checking dates on
other files that affect the content or appearance of documents, like
images.

Submitted by: grembo