doc problem xml

Routing-error-handlers.md

@@ -44,8 +44,10 @@ func index(ctx iris.Context) {
 
 Iris has builtin support for the [Problem Details for HTTP APIs](https://tools.ietf.org/html/rfc7807).
 
-The `Context.Problem` method sends a response like `Context.JSON` does but with indent of "  " and
-a content type of "application/problem+json" instead.
+The `Context.Problem` writes a JSON or XML problem response. Behaves exactly like `Context.JSON` but with default ProblemOptions.JSON indent of " " and a response content type of "application/problem+json" instead.
+
+Use the options.RenderXML and XML fields to change this behavior and
+send a response of content type "application/problem+xml" instead.
 
 ```go
 func newProductProblem(productName, detail string) iris.Problem {
@@ -72,6 +74,10 @@ func fireProblem(ctx iris.Context) {
         JSON: iris.JSON{
             Indent: "  ",
         },
+        // OR
+        // Render as XML:
+        // RenderXML: true,
+        // XML:       iris.XML{Indent: "  "},
         // Sets the "Retry-After" response header.
         //
         // Can accept:
@@ -105,4 +111,18 @@ func fireProblem(ctx iris.Context) {
 }
 ```
 
+When `RenderXML` is set to `true` then the response will look be rendered as XML instead.
+
+**Outputs "application/problem+xml"**
+
+```xml
+<Problem>
+    <Type>https://host.domain/product-error</Type>
+    <Status>400</Status>
+    <Title>Product validation problem</Title>
+    <Detail>problem error details</Detail>
+    <ProductName>product name</ProductName>
+</Problem>
+```
+
 Full example can be found at [_examples/routing/http-errors](https://github.com/kataras/iris/blob/master/_examples/routing/http-errors/main.go).