bug#33553: Document negatively listing fonts for character range

[積丹尼 Dan Jacobson]

(info "(emacs) Modifying Fontsets")
shows how to

+ Use a certain font for a certain range of characters

+ Prohibit a certain font for all characters

But it doesn't show how to

+ Prohibit a certain font for a certain range of characters

So to prohibit a certain font for just one character, the user must
prohibit it for all characters.

Or he must hardwire some other arbitrary font to be used for that one
character, he can not just say don't use a certain font for that
character.