docs/pwg/: Document base classes, update sections of n-to-1 and 1-to-n (muxer, fix...

docs/pwg/: Document base classes, update sections of n-to-1 and 1-to-n (muxer, fix some code examples and links and u...

Original commit message from CVS:
* docs/pwg/advanced-clock.xml:
* docs/pwg/appendix-porting.xml:
* docs/pwg/intro-preface.xml:
* docs/pwg/other-base.xml:
* docs/pwg/other-manager.xml:
* docs/pwg/other-nton.xml:
* docs/pwg/other-ntoone.xml:
* docs/pwg/other-oneton.xml:
* docs/pwg/pwg.xml:
Document base classes, update sections of n-to-1 and 1-to-n (muxer,
demuxer), remove n-to-n (was never written), fix some code examples
and links and update the porting section to include all this.