Improve doc strings in log-edit.el

Improve doc strings in log-edit.el

* lisp/vc/log-edit.el (log-edit-new-comment-index)
(log-edit-maximum-comment-ring-size, log-edit-previous-comment)
(log-edit-next-comment, log-edit-comment-search-backward)
(log-edit-comment-search-forward)
(log-edit-comment-to-change-log)
(log-edit-header-contents-regexp, log-edit-font-lock-gnu-style)
(log-edit, log-edit-mode, log-edit-fill-entry, log-edit-done)
(log-edit-kill-buffer, log-edit-insert-message-template)
(log-edit-insert-cvs-template, log-edit-insert-cvs-rcstemplate)
(log-edit-rewrite-fixes, log-edit-add-to-changelog)
(log-edit-generate-changelog-from-diff)
(log-edit-insert-changelog, log-edit-narrow-changelog)
(log-edit-changelog-entry, log-edit-changelog-insert-entries)
(log-edit-extract-headers): Enhance and reword doc strings.