DOC: Add note about form prevent_default behavior. (#1343)

* Add note about form prevent_default behavior.

* Make the default behavior clear on forms with attached event handlers.

docs/guide/src/en/interactivity/event_handlers.md

@@ -51,6 +51,8 @@ Any event handlers will still be called.
 
 > Normally, in React or JavaScript, you'd call "preventDefault" on the event in the callback. Dioxus does _not_ currently support this behavior. Note: this means you cannot conditionally prevent default behavior based on the data in the event.
 
+> Note about forms: if an event handler is attached to the `onsubmit` event of a form, default behavior is to **not submit it**, meaning having `prevent_default: "onsubmit"` will submit it in this case.
+
 ## Handler Props
 
 Sometimes, you might want to make a component that accepts an event handler. A simple example would be a `FancyButton` component, which accepts an `on_click` handler: