docs/design/part-block.txt: Further explain the use of flushing on blocked pads.

docs/design/part-block.txt: Further explain the use of flushing on blocked pads.

Original commit message from CVS:
* docs/design/part-block.txt:
Further explain the use of flushing on blocked pads.
* docs/gst/gstreamer-sections.txt:
* gst/gstpad.c: (gst_pad_is_blocking), (handle_pad_block),
(gst_pad_push_event):
* gst/gstpad.h:
Added new GstPadFlag : GST_PAD_BLOCKING.
Adds the notion of pads really blocking, which enables to properly
handle FLUSH_START/FLUSH_STOP events on blocked pads.
Fixes #358999
API: gst_pad_is_blocking()
API: GST_PAD_IS_BLOCKING() macro
API: GST_PAD_BLOCKING GstPadFlag