Fix bogus computation of setf expander sources.

2013-06-13  Didier Verna  <didier@didierverna.net>

	* src/item/symbol.lisp (source (setf-expander-defiition)):
	Properly handler the different types for the EXPANDER slot.
Improve rendering of short setf expanders documentation.

2013-06-13  Didier Verna  <didier@didierverna.net>

	* src/doc/symbol.lisp (render-@defsetf): Print "Expansion" instead
	of "Expands to" for short setf expander forms.