This is how we've always documented it. Postgres treats code points as "characters" not graphemes.
You don't need to go to anything as esoteric as emojis to see this either. Accented characters like é have no canonical forms that are multiple code points and in some character sets some accented characters can only be represented that way.
But I don't think there's any reason to consider changing e existing functions. They have to be consistent with substr and the other string manipulation functions.
We could add new functions to work with graphemes but it might bring more pain keeping it up to date....
