Improve generic accessors concatenation.

ChangeLog entries:

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

	* symbol.lisp (document): Only join generic reader and writer
	definitions when their docstrings are EQUAL.
Improve accessor methods concatenation.

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

	* symbol.lisp (document): Only join accessor reader and writer
	definitions when their docstrings are EQUAL.