Remove * in defcustom docstrings and tell

Remove * in defcustom docstrings and tell
for most options that customizing them doesn't affect buffers
already displaying footnotes.
(Footnote-refresh-footnotes, Footnote-renumber-footnotes): Fix
handling of text properties and identical start/end tags.  Do
not use format when renumbering.
(Footnote-set-style): Make it work.
(Footnote-insert-numbered-footnote): Simplify.
(Footnote-narrow-to-footnotes, Footnote-insert-footnote)
(Footnote-goto-footnote): Fix handling of empty section tag.
(Footnote-delete-footnote): Fix handling of identical start/end
tags, empty section tag, and spaced footnotes.  Do not use
kill-region.
(footnote-mode): Make most options buffer-local to avoid that
customizing messes up handling of footnotes in buffers that
already display them.