#1722 Documentation bug: Tk_PointToChar

[Joe English]

The description of Tk_PointToChar in TextLayout(3)
seems to indicate that the return value is always a
valid character index (except in the special case of an
empty string). It can actually return an index *one
past* the end of the string (even when the string is
nonempty). Manpage should be clarified to reflect this.

[Donal K. Fellows]

Fixed in HEAD. Can't be bothered to backport such a small docfix.