bug#24051: 24.5; doc of `external-debugging-output'

[Drew Adams]

> > But I do wonder how we should talk about the standard error
> > stream.  I see only one other occurrence of "standard error"
> > where it means the stream (and not the discussion of standard
> > Emacs errors)
> 
> That's strange, because I see it in no less than 7 other nodes of the
> ELisp manual, and most of those nodes mention it more than once.

You're right: Subprocess Creation, Synchronous Processes,
Asynchronous Processes, Output from Processes, Filter
Functions, Displaying Messages, and Batch Mode.

Not sure what I meant by the text you cite, but the rest
of what I wrote there should make clear that, although we
use the term in several places, we don't actually say what
we mean by it.  This bit, for instance:

 On the other hand, would it be good to say something about
 the streams (the terms) "standard output" and "standard error"
 in node `Output Streams'?  We refer to both (especially the
 former) in various places, but we don't say what those names
 mean.

Some short description, at least, would be helpful.