docs/pwg/advanced_interfaces.xml: Add documentation on propertyprobing.

docs/pwg/advanced_interfaces.xml: Add documentation on propertyprobing.

Original commit message from CVS:
2004-01-28  Ronald Bultje  <rbultje@ronald.bitfreak.net>

* docs/pwg/advanced_interfaces.xml:
Add documentation on propertyprobing.
* docs/pwg/advanced_events.xml:
* docs/pwg/advanced_tagging.xml:
* docs/pwg/building_boiler.xml:
* docs/pwg/building_filterfactory.xml:
* docs/pwg/pwg.xml:
Move filterfactory and tagging into their own chapter, add a chapter
on events. all these are empty placeholders that will be filled in
some day.