gst/gstelement.h: Clarify _NO_PREROLL a bit more.

gst/gstelement.h: Clarify _NO_PREROLL a bit more.

Original commit message from CVS:
* gst/gstelement.h:
Clarify _NO_PREROLL a bit more.
* gst/gstevent.c:
Fix docs.
* gst/gstpad.c: (gst_pad_link_check_hierarchy),
(gst_pad_get_caps_unlocked), (gst_pad_save_thyself),
(handle_pad_block), (gst_pad_push_event), (gst_pad_send_event):
Patch by: Yves Lefebvre <ivanohe at abacom dot com> Fix possible deadlock
due to wrong locking order. Fixes #361769.
Remove some redundant/misplaced checks in pad_block.
* libs/gst/base/gstbasesink.c: (gst_base_sink_get_position):
For negative rates, count backwards from the duration.