Fix up docstring conventions.

Fix up docstring conventions.
Move vars to before their first use.
(rst-mode): Don't mess with font-lock-support-mode.
(rst-suggest-new-decoration, rst-adjust-decoration):
Avoid CL's copy-list.
(rst-delete-entire-line): Use line-beginning-position.
(rst-position): New fun.
(rst-straighten-decorations): Use it instead of CL's position.
(rst-straighten-bullets-region): Avoid CL's mapcar*.
(rst-toc-mode): Use define-derived-mode.
(rst-iterate-leftmost-paragraphs, rst-iterate-leftmost-paragraphs-2):
Remove unused var `in-par'.  Use `point' rather than `point-marker'.
(rst-line-block-region): Reduce redundancy.  Use the `pfxarg' arg.
(rst-replace-lines): Simplify.