Documentation updates. Make BaseSink and BaseSrc docs contain the class structure so that people can actually see the...

Original commit message from CVS:
* docs/libs/gstreamer-libs-sections.txt:
* gst/gsttaglist.c:
* libs/gst/base/gstbasesink.c:
* libs/gst/base/gstbasesink.h:
* libs/gst/base/gstbasesrc.c:
* libs/gst/base/gstbasesrc.h:
Documentation updates. Make BaseSink and BaseSrc docs contain the
class structure so that people can actually see the prototypes for
virtual functions they're supposed to be overriding.