Fix bug#28557

Fix bug#28557

* test/lisp/emacs-lisp/cconv-tests.el: Remove `:expected-result :failed`
from the bug#28557 tests.
(cconv-tests-cl-function-:documentation): Account for the presence of
the arglist (aka "usage") in the docstring.

* lisp/emacs-lisp/cl-macs.el (cl--transform-lambda):
* lisp/emacs-lisp/cl-generic.el (cl-defgeneric):
Handle non-constant `:documentation`.

* lisp/emacs-lisp/generator.el (iter-lambda):
* lisp/emacs-lisp/cconv.el (cconv--convert-funcbody):
Use `macroexp-parse-body`.