bug#55086: 28.1; Improve doc of set-fontset-font

[Drew Adams]

> The eldoc string for set-fontset-font in Emacs 28 is
> 
>     (NAME TARGET FONT-SPEC &optional FRAME ADD)
> 
> I found NAME and TARGET to be unhelpfully generic.  The
> docstring does say that name is the name of a fontset and
> that TARGET is about characters.
> 
> The elisp manual names the params as:
> 
>  set-fontset-font name character font-spec &optional frame add
> 
> character is an improvement but it should be plural.
> 
> I think they should be the same in both places and
> this would be clearer:
> 
>     (FONTSET CHARACTERS FONT-SPEC &optional FRAME ADD)

+1 for that renaming.  (It's also OK to use just CHARS.)

The first doc line also doesn't really respect the
convention of describing args in order.  (Not a biggee,
IMO.)  That could maybe be remedied also - perhaps
something like this (67 chars):

 Modify FONTSET for use on CHARACTERS so that it respects FONT-SPEC.