bug#21644: 24.4; completing-read acts differently on functional collection

[Drew Adams]

> > If the bug is that a user will not know the details for a
> > function-valued COLLECTION argument to `completing-read' then the
> > right place to send a reader for __that information__ is node
> > `Programmed Completion'.
> 
> _This_ bug was about that. That doesn't mean we should fix it by
> introducing another bug.

No one has said anything that would introduce another bug.
It's about fixing this bug, by getting users to the information
about function-valued COLLECTION.  And that info is in node
`Programmed Completion'.

No one has said that users should not consult `Basic Completion'
for info about `all-completions', `try-completion', and
`test-completion'.

> > And I said, regarding general info about COLLECTION:
> >    It doesn't hurt to send them to both nodes or to only
> >    their parent, `Completion'.
> 
> I think that asking the user to read the whole Completion section is too
> much. 

No one is asking users to read the whole `Completion' section.
That is the section that covers all of the info about
`completing-read', _if_ you insist on pointing to only one section.

If you send them to only one section then a section that talks
only about the low-level functions `*-completion[s]' is not the
right destination.

Again: "It doesn't hurt to send them to both nodes".

> Basic Completion already contains the necessary information,

No, it does not.  It says *nothing* about `completing-read'.
Doesn't even mention it.

> or links to it.

As does `Completion'.

Really what should be done is to link from the `completing-read'
doc string only to node `Minibuffer Completion', which is the
node that documents `completing-read'.

But that node should document COLLECTION (it does not), and that
doc of COLLECTION should point to node `Programmed Completion'
for details about function-valued COLLECTION.

It is a bug (IMHO) that the node that documents `completing-read'
does not specify parameter COLLECTION - which is arguably the
*most important* parameter.

Fix that bug and link to node `Minibuffer Completion' from the
doc string, and you will have fixed this bug as well.