DOC: config: refer to section about quoting in the "add_item" converter

As requested by Nick in issue #1719, let's add a reference to the section
about quoting there, since add_item() will often be used with commas and
it's easy to mess up.

doc/configuration.txt

@@ -16873,7 +16873,8 @@ add_item(<delim>,[<var>][,<suff>]])
   sample string is empty before adding a delimiter. If commas or closing
   parenthesis are needed as delimiters, they must be protected by quotes or
   backslashes, themselves protected so that they are not stripped by the first
-  level parser. See examples below.
+  level parser (please see section 2.2 for quoting and escaping). See examples
+  below.
 
   Example:
     http-request set-var(req.tagged) 'var(req.tagged),add_item(",",req.score1,"(site1)") if src,in_table(site1)'