Oops, I left off the actual test result:
Test DEFCFUN.VARARGS.DOCSTRINGS failed Form: (DOCUMENTATION 'SPRINTF 'FUNCTION) Expected value: "sprintf docstring" Actual value: NIL.
Also:
$ clisp -q -norc [1]> (setf (documentation 'bozo 'function) "bozo docstring") NIL [2]> (documentation 'bozo 'function) NIL [3]> (defun foo (a b) "foo docstring" (+ a b)) FOO [4]> (documentation 'foo 'function) "foo docstring" [5]> (setf (documentation 'foo 'function) "changed foo docstring") "changed foo docstring" [6]> (documentation 'foo 'function) "changed foo docstring" [7]>