bug#19767: 25.0.50; cl-defgeneric docs

[Stefan Monnier]

> I see lisp/emacs-lisp/cl-generic.el was added recently, and that's great.
> But the CL manual hasn't been updated, I think?

Indeed, that's the main missing part of cl-generic right now.

> I could try to help here, but I haven't used this facility before so I
> need some pointers.

cl-generic tries to implement generic function part of CLOS (CLOS
is the set of object-oriented features of Common-Lisp, and it includes
the definition of objects&classes as well as the definition of methods
which use the "generic function" approach).  I tried to follow CLOS
pretty closely for all the things where CLOS seemed sane (i.e. that
excludes method-combinations).

So there's existing docs ain the form of the "Common Lisp Hyperspec",
such as http://www.lispworks.com/documentation/HyperSpec/Body/07_fb.htm
but the copyright status of this is kind of murky (for example, the GCL
doc includes it, but Debian took it out from its gcl-doc package because
of this legal uncertainty), so we can't just use that, sadly.


        Stefan