font for jit chars?

[Ilya N. Golubev]

`specifier-matching-instance', even if not signals error, no longer
returns font actually used for displaying emchars in
`jit-ucs-charset-0'.  One it returns (as part of font instance) just
does not have the code points that are still displayed.  How do we
figure the font used for these emchars?  How do we customize it?
Please document.