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

[Drew Adams]

> > > It is a bug (IMHO) that the node that documents `completing-read'
> > > does not specify parameter COLLECTION - which is arguably the
> > > *most important* parameter.
> 
> Done.
> I also re-added the references to the completion functions into the
> doc string of completing-read (and added 2 more functions to that
> list).  While referencing the manual is good, especially for complex
> topics, I feel that the absolute minimum of usage information,
> including the arguments with which callbacks can be called, should be
> available from the doc strings.

Great.  Thx.