bug#19722: 25.0.50; (elisp) `Tool Bar': Add xref to doc for image specifications

[Drew Adams <drew.adams@oracle.com>]

> > I note too that neither ":image" nor "image spec" has a substring match
> > for `i'.  (Yes, I know that you will reply that the former cannot be
> > indexed - dommage.)
> >
> > FWIW, these (attached) are the only index entries I see when matching
> > "image" as a substring.  Dunno where to find the info about "image
> > specifications."  (Even searching for ":image" did not seem to help.)
> 
> Please tell what information you were looking for, and which phrases
> went through your head when you looked for that.

I was trying to find what an "image specification" is - its
format/structure etc.  One cannot understand the doc for `:image' in
that node without knowing what "image specification" amounts to.

Perhaps "image format" or "image descriptor" is the same thing? If so
then it could help to use the same terminology throughout, but an xref
is still needed here, IMO.  Perhaps it should go to node `Image Formats'
or to node `Image Descriptors'.  Dunno.

The term "image specification" is used in the manual in many places,
but none of those places seem to define/describe what it is.  Perhaps
each of those occurrences should be changed to a different term (?).
And perhaps each of those occurrences should have an xref to wherever
this thingie is described.