docs/pwg/: General placeholders for now.

docs/pwg/: General placeholders for now.

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

* docs/pwg/advanced_clock.xml:
* docs/pwg/advanced_interfaces.xml:
* docs/pwg/advanced_midi.xml:
General placeholders for now.
* docs/pwg/advanced_request.xml:
Explanation about sometimes and request pads.
* docs/pwg/advanced_scheduling.xml:
Concept of bytestream, loopfunctions and schedulers.
* docs/pwg/building_boiler.xml:
Add something about plugin-init.