Changelog:

* imenu.el (imenu--subalist-p): Don't error on non-conses and
allow non-lambda lists as functions.

In my opinion a predicate should not throw an error.  Anyway more
important is the `non-lambda' lists part.

(imenu--index-alist): Don't recurse into non-subalists.

Looking at the commentary, the ability to embed functions in a menu has
clearly been added after this functions was written.

(imenu): Don't pass function itself as an argument.

,----[ C-h v (describe-variable 'imenu-generic-expression) RET ]
| Each element of this list should have the form
| 
|   (MENU-TITLE REGEXP INDEX [FUNCTION] [ARGUMENTS...])
| 
| FUNCTION, if present, specifies a function to call when the index
| item is selected by the user.  This function is called with
| arguments consisting of the item name, the buffer position, and
| the ARGUMENTS.
`----

(cddr index-item) == (FUNCTION . ARGS), is wrong as argument list, we
need one more cdr.

* doc/lispref/modes.texi: Make it clear that sub-alist is the cdr.

-ap