Improve documentation about Comment filter (#34539)

Include a mention to the decoration option, which allows someone to
specify the character comment to be used.

docs/docsite/rst/playbooks_filters.rst

@@ -666,6 +666,17 @@ Similar way can be applied style for C (``//...``), C block
     {{ "Erlang style" | comment('erlang') }}
     {{ "XML style" | comment('xml') }}
 
+If you need a specific comment character that is not included by any of the
+above, you can customize it with::
+
+  {{ "My Special Case" | comment(decoration="! ") }}
+
+producing::
+
+  !
+  ! My Special Case
+  !
+
 It is also possible to fully customize the comment style::
 
     {{ "Custom style" | comment('plain', prefix='#######\n#', postfix='#\n#######\n   ###\n    #') }}