DOC: Update delimiter param description.

[rossbar]

Update the description for the delimiter parameter in loadtxt to note that only single-character strings are supported. In principle, this new info could be in a .. versionchanged:: 1.23 directive - any preference there @seberg ?

Closes #22311

[seberg]

Yeah, I don't have a a real preference, but maybe it is good to be clear about the change. We also disallowed \n and \r (newline characters). (Now I remember that is because newlines are a no-op in the normal path, but in theory not if you consume iterables of lines rather than files.)

[seberg]

Thanks, Ross.

[rossbar]

@charris I've added the backport candidate label for this one even though it's a docs PR. The change went into effect in 1.23 so the notification of the change in the docs should in principle be backported as well. It's not critically important though - apologies if this is an abuse of the backport candidate label!