Doc fixes.

Doc fixes.
(comint-last-input-start, comint-last-input-end)
(comint-last-output-start): Give them values.
(comint-arguments): Avoid warning.
(comint-skip-prompt): Use line-end-position.
(comint-backward-matching-input): Rename arg to agree with doc.
(comint-extract-string): Use syntax-ppss.
(comint-dynamic-simple-complete): Delete useless list construction.
(comint-redirect-subvert-readonly): New.
(comint-redirect-preoutput-filter): Use it.