bug#2949: 23.0.92; font-info incompatible across Emacs versions

[Drew Adams]

> new display engine for the release of 21.
> Before that, fonts are specified for each charset,
> but after that, fonts are specified for each character
> using a char table.  As a result, the concept for charset
> for a font lost the meaning.

Thanks for the explanation. 

> I fixed the docstring.

Thanks.