Fix doc strings in ispell.el/.

Fix doc strings in ispell.el/.

 lisp/textmodes/ispell.el (ispell-skip-tib, ispell-keep-choices-win)
 (ispell-choices-win-default-height, ispell-silently-savep)
 (ispell-dictionary-alist, ispell-encoding8-command)
 (ispell-check-version, ispell-aspell-find-dictionary)
 (ispell-valid-dictionary-list, ispell-words-keyword)
 (ispell-get-word, ispell-internal-change-dictionary)
 (ispell-region, ispell-skip-region-list)
 (ispell-begin-skip-region-regexp, ispell-ignore-fcc)
 (ispell-process-line, ispell-minor-mode, ispell-minor-check)
 (ispell-message-text-end, ispell-message)
 (ispell-buffer-local-parsing): Doc fix.