minor #3400 Add another paragraph about mentioning another needed lib…

…rary (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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4584576 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.