Rename gst_collecpads_foo() => gst_collect_pads_foo(). Document unimplemented functions as unimplemented (#320766).

Original commit message from CVS:
* docs/gst/gstreamer-sections.txt:
* win32/common/libgstbase.def:
* win32/libgstbase.def:
* gst/base/gstcollectpads.c: (gst_collect_pads_base_init),
(gst_collect_pads_class_init), (gst_collect_pads_init),
(gst_collect_pads_finalize), (gst_collect_pads_new),
(gst_collect_pads_set_function), (gst_collect_pads_add_pad),
(gst_collect_pads_remove_pad), (gst_collect_pads_is_active),
(gst_collect_pads_collect), (gst_collect_pads_collect_range),
(gst_collect_pads_start), (gst_collect_pads_stop),
(gst_collect_pads_peek), (gst_collect_pads_pop),
(gst_collect_pads_available), (gst_collect_pads_read),
(gst_collect_pads_flush), (gst_collect_pads_event),
(gst_collect_pads_chain):
* gst/base/gstcollectpads.h:
Rename gst_collecpads_foo() => gst_collect_pads_foo(). Document
unimplemented functions as unimplemented (#320766).