(checkdoc-display-status-buffer): Fix docstring.
(checkdoc-interactive, checkdoc-message-interactive): Make them perform spell
checking when appropriate.
(checkdoc-interactive-loop): Fix docstring and a few typos.
(checkdoc-interactive-ispell-loop)
(checkdoc-message-interactive-ispell-loop): New functions.
(checkdoc-next-error): Fix docstring.
(checkdoc-this-string-valid-engine): Fix typo.
(checkdoc-ispell-docstring-engine): Do test for checkdoc-autofix-flag = nil.