docs/pwg/advanced-types.xml: Well done to Michael for catching my deliberate introduction of this spelling mistake.

Original commit message from CVS:
* docs/pwg/advanced-types.xml:
Well done to Michael for catching my deliberate introduction
of this spelling mistake.
* gst/gstbin.c: (gst_bin_remove_func), (bin_bus_handler):
* gst/gstelement.h:
Add GST_ELEMENT_UNPARENTING to prevent races so that we can
unlink pads before removing the element from the bin.