bug#22336: 25.0.50; cl-generic.el features are not documented in ELisp manual

[Stefan Monnier]

> (cl-defgeneric foo (a)
>   (:method ((s string)) "stringy!")
>   (:method ((a array)) :four)
>   10)

I implemented this :method thingy simply because CLOS allows it, but I'm
not sure it's worth documenting.

> But the docstrings don't mention any of the possible values of TYPE (except
> (eql ...)), not just "plain old" ones. So I'm not clear on what exactly you
> consider an omission here. cl-defmethod docstring should probably enumerate
> the possible types (aside from the mentioned ones, TYPE can be (head ...)
> or a name of cl-struct, like the commentary says).

Actually, the only "builtin" specializer is t (the generic one).
All other ones are defined via cl-generic-define-generalizer, and we
should improve cl-defmethod's docstring so that it automatically
mentions all the specializers that happen to be currently defined
(i.e. in a dynamic way, just like pcase's docstring dynamically lists
the patterns currently defined via pcase-defmacro).


        Stefan