Undocumented predicates

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

 read_to_end(Arg1, Arg2)

 read_to_end(Arg1, Arg2, Arg3)

 get_line(Arg1, Arg2)

 get_line(Arg1)

 line(Arg1)

 write_string(Arg1, Arg2)

 write_string(Arg1)

 file_to_string(Arg1, Arg2)

 file_to_string(Arg1, Arg2, Arg3)

 string_to_file(Arg1, Arg2)

 output_to_file(Arg1, Arg2)

 open_input(Arg1, Arg2)

 close_input(Arg1)

 open_output(Arg1, Arg2)

 close_output(Arg1)