add option to define opening and closing of org export letter

lisp/my/my-org-article.el

@@ -568,6 +568,7 @@ IMPORTANT en version must be defined")
   "[paper=a4,fontsize=11pt,DIV=12,BCOR=10mm,headsepline,<<documentclass>>,captions=tableheading,version=first,bibliography=totoc,dvipsnames]"
   "The default LaTeX class options.
 
+Replaces <<...>> with values defined in
 `my-org-article-latex-lang-template-plist'"
   :group 'my-org-export-article
   :type '(string :tag "LaTeX class"))
@@ -2084,6 +2085,7 @@ IMPORTANT en version must be defined")
 
 "
   "Preamble options.
+Text like <<...>>> will be replaced according to
 `my-org-article-latex-lang-template-plist'"
   :group 'my-org-export-article)