Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 pchar(Arg1)

 pchar_upper(Arg1, Arg2)

 pchar_lower(Arg1, Arg2)

 ch_map(Arg1, Arg2, Arg3)

 ch(Arg1, Arg2, Arg3)

 pchar_type(Arg1, Arg2)

 pchar_code(Arg1, Arg2)

 pdif_char(Arg1, Arg2, Arg3)