bug#21960: 25.1.50; add info binding to copy node as @xref

[Tom Tromey]

Xue> There are many ways to add a cross reference: @xref (starts a
Xue> sentence), @ref (at the end of a sentence), @pxref (within
Xue> parentheses) etc.

Xue> And the arguments are not immutable.  Apart from the node name, it can
Xue> also contain the cross reference name (although usually omitted),
Xue> section name (used in printed manual), Info file name (if it is
Xue> different from the current file, for example, an @pxref from
Xue> doc/lispref/ to doc/emacs/) and printed manual name.

Xue> Do you want a general command or a specific command[1]?

Yeah, good question.

The main problem I have is that when I go to make a cross-manual
reference, I don't remember the syntax -- which argument is the file
name, etc.  I end up having to look it up every time.

So I guess something that copied the node as
@xref{blah, , , file} would be good for me; but I don't know if that
is idiosyncratic or helpful to anybody else.

Another way to address the underlying issue would be to enable eldoc for
texinfo mode.

Tom