doc the optional ProblemOptions

Routing-error-handlers.md

@@ -65,7 +65,31 @@ func newProductProblem(productName, detail string) iris.Problem {
 }
 
 func fireProblem(ctx iris.Context) {
-    ctx.Problem(newProductProblem("product name", "problem error details"))
+    // Response like JSON but with indent of "  " and
+    // content type of "application/problem+json"
+    ctx.Problem(newProductProblem("product name", "problem error details"), iris.ProblemOptions{
+        // Optional JSON renderer settings.
+        JSON: iris.JSON{
+            Indent: "  ",
+        },
+        // Sets the "Retry-After" response header.
+        //
+        // Can accept:
+        // time.Time for HTTP-Date,
+        // time.Duration, int64, float64, int for seconds
+        // or string for date or duration.
+        // Examples:
+        // time.Now().Add(5 * time.Minute),
+        // 300 * time.Second,
+        // "5m",
+        //
+        RetryAfter: 300,
+        // A function that, if specified, can dynamically set
+        // retry-after based on the request. Useful for ProblemOptions reusability.
+        // Overrides the RetryAfter field.
+        //
+        // RetryAfterFunc: func(iris.Context) interface{} { [...] }
+    })
 }
 ```