Prepare for rewrite of ordinary functions documentation merging.

2013-06-19  Didier Verna  <didier@didierverna.net>

	* src/doc/symbol.lisp (document (accessor-definition)): Temporarily
	disable documentation merging. Reference writer in accessor
	documentation.
Minor code cleanup.

2013-06-19  Didier Verna  <didier@didierverna.net>

	* src/doc/symbol.lisp (funcoid-definition): Improve docstring.
	(add-symbol-definition): Rename some local variables.
First step to accessors cross-references.

Readers and writers now point back to each other (even heterogeneous
ones, that is, when one is an ordinary function and the other a generic
one). Ordinary accessors now render those backlinks (generic ones not yet).

2013-06-19  Didier Verna  <didier@didierverna.net>

	* src/item/symbol.lisp (writer-definition):
	(generic-writer-definition): New READER slots.
	(find-definition (:accessor)):
	(find-definition (:generic-accessor)):
	(category-definitions (:accessor)):
	(category-definitions (:writer)):
	(category-definitions (:generic-accessor)):
	(category-definitions (:generic-writer)): New methods.
	(add-symbol-definition): Create heterogeneous (generic)
	accessors. Fill in writer's READER slot in the homogeneous case.
	(finalize-definitions): Fill in READER and WRITER slots in
	heterogeneous accessors.
	* src/doc/symbol.lisp (document(writer-definition)): New method.
	(document (accessor)): Render ordinary writer documentation only.