bug#56401: "Case folding" should be called "ignore case"

[Po Lu]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> In any case, I think renaming all the functions and variables here would
> not be worth the churn -- there's quite a lot of them.

+1.  For what it's worth, the glossary in the Emacs manual also tells
you what "Case Folding" means.