You can pass "attributes" (extra data) on your API calls that can be used within your emails similarly to how you would normally pass any attribute for a subscriber.
Depending on what type of additional information you're trying to include, within your email that is being triggered, you would need to include a personalization string like, %%attributename1%%. This will then utilize the Value you're passing at the time of send. If this attribute doesn't exist within the "Profile Management" section (under Subscribers tab) in your account, you can also use a data extension with this field/attribute name and assign it to this triggered send definition. This will allow the email to pass validation upon saving your triggered send definition.
The XML Attribute you're referring to is just a manually created attribute on the account that is given as an example. They are using AMPScript to Build the email/Rowset from XML formatted data. You may not need to use this to accomplish what you're doing and you can create and name your attribute (or Data Extension field name) anything you want. You would then just need to pass it as an attribute when making your API call similar to the below.
<Value>This value will be passed to my email at the time the email is triggered</Value>
Your email/HTML would look similar to the below as well and once the email is triggered, %%attributename1%% would be replaced with the value defined above.
Here's our test triggered send email and everything below is passed through my API call.
Some quick info below as well which gives details on "HTML Attributes" if you're looking to pass HTML code as an attribute. Hopefully this is helpful and clear. Let me know if you have any questions.