bug#32401: Improving set-fontset-font docstring

[Arun Isaac]

There is a minor grammatical error in the following part of the
docstring of set-fontset-font.

"TARGET may be nil.  In that case, use FONT-SPEC for any characters for
that no FONT-SPEC is specified."

It should be "for which no FONT-SPEC is specified" instead of "for that
no FONT-SPEC is specified". Replace "that" by "which".

Also, in an earlier part of the docstring, the description of "script
name symbol" and "charset" could be improved by adding a link to the
list of available scripts and charsets, or describing some means to find
the supported scripts and charsets.