bug#12456: 24.2.50; Completion in `Info-goto-node' (cross-manual jump)

[Stefan Monnier <monnier@IRO.UMontreal.CA>]

> Recipe from emacs -Q: C-h r g ( e l i s p ) n n <TAB>
> When I do it, I see in the echo area the message "[Complete, but not
> unique]", which is wrong, as the elisp manual has no node called (nor
> beggining with) "nn".
> OTOH, if I repeat the experiment but this time with "(elisp)num",
> Emacs says the same message (this time is right), but a second <TAB>
> produces the message "[No completions]", which is wrong, as there is 3
> nodes in the elisp manual whose names start with "num".

IIRC the core of the problem is that Info's completion table does not
(yet) know how to do completion after something like "(elisp)", so it
basically returns some "dummy" completion data (because the way
completion tables are defined currently, they have no way to say "I
don't know").

So there are 2 ways to fix the above problem:
- Extend minibuffer.el so a completion table return "don't know" (at
  which point it could put a message like " [No completion info]").
- Extend info.el so that it does provide actual completion by opening up
  the "elisp" info file and gathering the corresponding node names.


        Stefan