Add WithDescription method to Function struct

Extend the `Function` struct with a `WithDescription` method to allow setting descriptions fluently.

function.go

@@ -47,6 +47,11 @@ func (f Function) WithSyntheticFields(fieldsAndDescriptions map[string]string) F
 	return f
 }
 
+func (f Function) WithDescription(description string) Function {
+	f.Description = description
+	return f
+}
+
 func (f Function) Execute(ctx *Context, input string) (any, error) {
 	if !f.fn.IsValid() {
 		return "", fmt.Errorf("function %s is not implemented", f.Name)