unicode: various documentation tweaks

unicode: various documentation tweaks

The comment on IsOneOf regarding Latin-1 was an implementation detail:
when the function is called internally, that condition is true. It used to matter,
but now the comment is a dreg. The function works fine if the character is
Latin-1, so we just delete the comment.

Fixes #2966.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5655047