minor #3400 Add another paragraph about mentioning another needed library (OskarStark)

This PR was submitted for the 3.x branch but it was squashed and merged into the 2.x branch instead.

Discussion
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Add another paragraph about mentioning another needed library

Commits
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4584576c Add another paragraph about mentioning another needed library

doc/filters/markdown_to_html.rst

@@ -27,11 +27,13 @@ removed consistently before conversion:
         Hello!
     {% endapply %}
 
-You can also use the filter on an included file:
+You can also use the filter on an included file or a variable:
 
 .. code-block:: twig
 
     {{ include('some_template.markdown.twig')|markdown_to_html }}
+    
+    {{ changelog|markdown_to_html }}
 
 .. note::
 
@@ -63,3 +65,6 @@ You can also use the filter on an included file:
                 }
             }
         });
+       
+    Afterwards you need to install a markdown library of your choice. Some of them are
+    mentioned in the ``require-dev`` section of the ``twig/markdown-extra`` package.