gst/: Small cleanups in docs and code.

gst/: Small cleanups in docs and code.

Original commit message from CVS:
* gst/gstobject.c: (gst_object_set_parent):
* gst/gstpipeline.c: (do_pipeline_seek):
Small cleanups in docs and code.
* gst/gstsegment.c: (gst_segment_clip):
* tests/check/gst/gstsegment.c: (GST_START_TEST):
if stop == start and start is in the segment, no clipping should be
done. Also add a test for this.