[Backport 7.x] The agent function should take the Connection contructor options as argument (#1334)

Co-authored-by: Tomas Della Vedova <delvedor@users.noreply.github.com>

docs/configuration.asciidoc

@@ -149,7 +149,9 @@ const client = new Client({
 
 const client = new Client({
   node: 'http://localhost:9200',
-  agent: () => new CustomAgent()
+  // the function takes as parameter the option
+  // object passed to the Connection constructor
+  agent: (opts) => new CustomAgent()
 })
 
 const client = new Client({