Support interactive editing of comment blocks

* lisp/org-src.el (org-edit-comment-block): New command to edit
comment block elements.  The command auto-escapes Org markup inside.
(org-src--contents-area):
* lisp/org.el (org-insert-structure-template):
(org-edit-special): Support comment blocks.
* etc/ORG-NEWS (Interactive commands now support escaping text inside
comment blocks):
(New command ~org-edit-comment-block~ to edit comment block at point):
Document the new features.

See https://orgmode.org/list/87y1wc3ruw.fsf@mat.ucm.es

etc/ORG-NEWS

@@ -141,6 +141,14 @@ discouraged when working with Org files.
 
 ** New features
 
+*** Interactive commands now support escaping text inside comment blocks
+
+~org-edit-special~ and ~org-insert-structure-template~ now handle
+comment blocks.
+
+See [[*New command ~org-edit-comment-block~ to edit comment block at
+point]].
+
 *** New customization option =org-property-separators=
 A new alist variable to control how properties are combined.
 
@@ -301,6 +309,16 @@ inside entry.  This was the default previously.
 
 Now, ~org-fold-show-entry~ does not fold drawers by default.
 
+*** New command ~org-edit-comment-block~ to edit comment block at point
+
+As the contents of comments blocks is not parsed as Org markup, the
+headlines and keywords inside should be escaped, similar to src
+blocks, example blocks, and export blocks.  This in inconvenient to do
+manually and ~org-edit-special~ is usually advised to edit text in
+such kind of blocks.
+
+Now, comment block editing is also supported via this new function.
+
 *** New function ~org-element-cache-map~ for quick mapping across Org elements
 
 When element cache is enabled, the new function provides the best
@@ -314,7 +332,6 @@ to ~org-element--cache-map-statistics~ and
 ~org-element--cache-map-statistics-threshold~.
 
 ~org-scan-tags~ and tag views in agenda utilise the new function.
-
 *** New function ~org-element-at-point-no-context~
 
 This function is like ~org-element-at-point~, but it does not try to

lisp/org-src.el

@@ -384,7 +384,7 @@ where BEG and END are buffer positions and CONTENTS is a string."
        (let ((beg (org-element-property :contents-begin datum))
 	     (end (org-element-property :contents-end datum)))
 	 (list beg end (buffer-substring-no-properties beg end))))
-      ((memq type '(example-block export-block src-block))
+      ((memq type '(example-block export-block src-block comment-block))
        (list (progn (goto-char (org-element-property :post-affiliated datum))
 		    (line-beginning-position 2))
 	     (progn (goto-char (org-element-property :end datum))
@@ -1161,6 +1161,29 @@ Throw an error when not at an export block."
        (lambda () (org-escape-code-in-region (point-min) (point-max)))))
     t))
 
+(defun org-edit-comment-block ()
+  "Edit comment block at point.
+\\<org-src-mode-map>
+A new buffer is created and the block is copied into it, and the
+buffer is switched into Org mode.
+
+When done, exit with `\\[org-edit-src-exit]'.  The edited text \
+will then replace the area in the Org mode buffer.
+
+Throw an error when not at a comment block."
+  (interactive)
+  (let ((element (org-element-at-point)))
+    (unless (and (eq (org-element-type element) 'comment-block)
+		 (org-src--on-datum-p element))
+      (user-error "Not in a comment block"))
+    (org-src--edit-element
+     element
+     (org-src--construct-edit-buffer-name (buffer-name) "org")
+     'org-mode
+     (lambda () (org-escape-code-in-region (point-min) (point-max)))
+     (org-unescape-code-in-string (org-element-property :value element)))
+    t))
+
 (defun org-edit-src-code (&optional code edit-buffer-name)
   "Edit the source or example block at point.
 \\<org-src-mode-map>

lisp/org.el

@@ -8873,7 +8873,8 @@ When foo is written as FOO, upcase the #+BEGIN/END as well."
 	 (region-end (and region? (copy-marker (region-end))))
 	 (extended? (string-match-p "\\`\\(src\\|export\\)\\'" type))
 	 (verbatim? (string-match-p
-		     (concat "\\`" (regexp-opt '("example" "export" "src")))
+		     (concat "\\`" (regexp-opt '("example" "export"
+                                                "src" "comment")))
 		     type))
          (upcase? (string= (car (split-string type))
                            (upcase (car (split-string type))))))
@@ -16998,6 +16999,7 @@ When at a table, call the formula editor with `org-table-edit-formulas'.
 When in a source code block, call `org-edit-src-code'.
 When in a fixed-width region, call `org-edit-fixed-width-region'.
 When in an export block, call `org-edit-export-block'.
+When in a comment block, call `org-edit-comment-block'.
 When in a LaTeX environment, call `org-edit-latex-environment'.
 When at an INCLUDE, SETUPFILE or BIBLIOGRAPHY keyword, visit the included file.
 When at a footnote reference, call `org-edit-footnote-reference'.
@@ -17044,6 +17046,7 @@ Otherwise, return a user error."
       (`table-row (call-interactively 'org-table-edit-formulas))
       (`example-block (org-edit-src-code))
       (`export-block (org-edit-export-block))
+      (`comment-block (org-edit-comment-block))
       (`fixed-width (org-edit-fixed-width-region))
       (`latex-environment (org-edit-latex-environment))
       (`planning