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

[Howard Melman]

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:

    Function: 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)

Howard