docs/random/moving-plugins: Make it clear that the "compiled-in descriptions" really mean the element details.

Original commit message from CVS:
* docs/random/moving-plugins:
Make it clear that the "compiled-in descriptions" really mean
the element details.
* libs/gst/base/gstbasesink.c: (gst_base_sink_commit_state),
(gst_base_sink_wait_preroll):
Update docs.
* docs/libs/gstreamer-libs-sections.txt:
* libs/gst/base/gstbasesrc.c: (gst_base_src_wait_playing),
(gst_base_src_get_range), (gst_base_src_activate_push):
* libs/gst/base/gstbasesrc.h:
Added function to block while waiting for PLAYING, this function
is used by live sources that block on the clock.
API: gst_base_src_wait_playing()