gst/base/gstbasesink.c: Add comment.

gst/base/gstbasesink.c: Add comment.

Original commit message from CVS:
* gst/base/gstbasesink.c: (gst_base_sink_pad_buffer_alloc):
Add comment.
* gst/elements/gstfakesink.c: (gst_fake_sink_init),
(gst_fake_sink_change_state):
Make state change function thread-safe.
* gst/gstpad.c: (gst_pad_alloc_buffer):
Set offset on generic buffer allocated by fallback.