Actually document #wontfix

[Kapendev]

On Monday, 28 July 2025 at 19:27:24 UTC, monkyyy wrote:
> Why that sounds like indexOf, herp derp, every user better go 
> learn the hardway why it isnt named that. Its SO fun to have an 
> off by one error when a user does some unicode

Most people will find and use `indexOf` first when searching for 
string functions, so this avoids some confusion. A small comment 
on `countUntil` about unicode could be helpful though. Or maybe 
the description of algorithm could include a comment.