Function signature should read advertised-calling-convention

[DamienCassou]

Some functions have an advertised-calling-convention to specify their signature, e.g., start-process-shell-command:

(defun start-process-shell-command (name buffer &rest args)
  "Start a program in a subprocess.  Return the process object for it.
[...]"
  (declare (advertised-calling-convention (name buffer command) "23.1"))

In these cases, describe-function will display the advertised signature whereas helpful will display the implemented signature.