* lisp/textmodes/rst.el: Minor cleanup to improve style.

* lisp/textmodes/rst.el: Minor cleanup to improve style.
(rst-get-decoration): Eliminate unneeded assignment.
(rst-update-section, rst-promote-region, rst-straighten-decorations)
(rst-section-tree, rst-adjust): Use point-marker.
(rst-toc-mode-mouse-goto): Avoid setq.
(rst-shift-region-guts, rst-shift-region-left)
(rst-iterate-leftmost-paragraphs, rst-iterate-leftmost-paragraphs-2)
(rst-convert-bullets-to-enumeration): Use copy-marker.