Rename audiovoice to audiokaraoke and add it to the docs.

Rename audiovoice to audiokaraoke and add it to the docs.

Original commit message from CVS:
* docs/plugins/Makefile.am:
* docs/plugins/gst-plugins-good-plugins-docs.sgml:
* docs/plugins/gst-plugins-good-plugins-sections.txt:
* docs/plugins/gst-plugins-good-plugins.args:
* docs/plugins/gst-plugins-good-plugins.hierarchy:
* docs/plugins/inspect/plugin-audiofx.xml:
* gst/audiofx/Makefile.am:
* gst/audiofx/audiofx.c:
* gst/audiofx/audiokaraoke.c:
* gst/audiofx/audiokaraoke.h:
* gst/audiofx/audiovoice.c:
* gst/audiofx/audiovoice.h:
Rename audiovoice to audiokaraoke and add it to the docs.