docs/pwg/advanced_types.xml: Add notes on creating your own types.

docs/pwg/advanced_types.xml: Add notes on creating your own types.

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

* docs/pwg/advanced_types.xml:
Add notes on creating your own types.
* docs/pwg/building_boiler.xml:
* docs/pwg/building_pads.xml:
* docs/pwg/building_state.xml:
Add some stuff about how to retrieve values from structures, how
that relates to types and change layout slightly again to be almost
perfect.