docs/manual/: Fix playbin/decodebin examples, update docs a bit, mention bus instead of signals in various places, me...

Original commit message from CVS:
* docs/manual/advanced-autoplugging.xml:
* docs/manual/appendix-checklist.xml:
* docs/manual/appendix-integration.xml:
* docs/manual/highlevel-components.xml:
Fix playbin/decodebin examples, update docs a bit, mention bus
instead of signals in various places, mention kmplayer and
kaffeine since they have a working GStreamer backend in the KDE
section.