Add "commentpragma" html style rule (#5726)

* html-comment, that can be used in the pragma area

* add commentpragma test

* fix typo

* fix typo and change comments ab bit

* combine html-comment and pragma-comment and add some docs, how to use it

* Make docs simpler by removing caching info

* change h2 wording

core/modules/parsers/wikiparser/rules/commentblock.js

@@ -7,6 +7,12 @@ Wiki text block rule for HTML comments. For example:
 
 ```
 <!-- This is a comment -->
+\define macroX()
+<!-- This is a comment -->
+xxxx
+\end
+<!-- This is a comment -->
+
 ```
 
 Note that the syntax for comments is simplified to an opening "<!--" sequence and a closing "-->" sequence -- HTML itself implements a more complex format (see http://ostermiller.org/findhtmlcomment.html)
@@ -19,7 +25,7 @@ Note that the syntax for comments is simplified to an opening "<!--" sequence an
 "use strict";
 
 exports.name = "commentblock";
-exports.types = {block: true};
+exports.types = {block:true, pragma:true};
 
 exports.init = function(parser) {
 	this.parser = parser;