lisp/org.el: Display subtitle lines with document info faces

* lisp/org.el: The #+SUBTITLE: keyword is now displayed with
`org-document-info-keyword' and its value with
`org-document-info' (instead of `org-meta-line').
* lisp/org-faces.el (org-document-info):
(org-document-info-keyword): Update docstrings to include #+SUBTITLE:.

Like the title, author, e-mail, date, the subtitle is information
that is displayed in the exported document, not meta information.

lisp/org-faces.el

@@ -414,12 +414,14 @@ changes."
   '((((class color) (background light)) (:foreground "midnight blue"))
     (((class color) (background dark)) (:foreground "pale turquoise"))
     (t nil))
-  "Face for document date, author and email; i.e. that which
-follows a #+DATE:, #+AUTHOR: or #+EMAIL: keyword."
+  "Face for document subtitle, date, author and email; i.e. that
+which follows a #+SUBTITLE:, #+DATE:, #+AUTHOR: or #+EMAIL:
+keyword."
   :group 'org-faces)
 
 (defface org-document-info-keyword '((t :inherit shadow))
-  "Face for #+TITLE:, #+AUTHOR:, #+EMAIL: and #+DATE: keywords."
+  "Face for #+TITLE:, #+SUBTITLE:, #+AUTHOR:, #+EMAIL: and
+#+DATE: keywords."
   :group 'org-faces)
 
 (defface org-block `((t :inherit shadow

lisp/org.el

@@ -5307,7 +5307,7 @@ by a #."
 		 (min (point-max) end-of-endline))
 	       '(face org-block-end-line)))
 	    t))
-	 ((member dc1 '("+title:" "+author:" "+email:" "+date:"))
+	 ((member dc1 '("+title:" "+subtitle:" "+author:" "+email:" "+date:"))
 	  (org-remove-flyspell-overlays-in
 	   (match-beginning 0)
 	   (if (equal "+title:" dc1) (match-end 2) (match-end 0)))