options, corrected docs for evaluator trap options.
* scheme-evaluation.texi (Scheme Read): New docs for read-options,
read-enable, read-disable and read-set! and cross references to
option nodes.
(Evaluator Options): New docs for eval-options, eval-enable,
eval-disable and eval-set!, traps, trap-enable, trap-disable and
trap-set! and cross references to option nodes.
(Evaluator Behaviour): Renamed node from `Evaluator options' to
avoid name clash.
* scheme-io.texi (String Ports): Added docs for SRFI-6 procedures.
(Void Ports): Corrected introductory comment.
