Document the turbo-frame[complete] attribute

Explain the `[complete]` attribute's behavior introduced in
[hotwired/turbo#487][].

[hotwired/turbo#487]: hotwired/turbo#487

_source/handbook/03_frames.md

@@ -104,6 +104,9 @@ Note that the `<turbo-frame>` on `/emails/set_aside` does not contain a `src` at
 
 During navigation, a Frame will set `[aria-busy="true"]` on the `<turbo-frame>` element when fetching the new contents. When the navigation completes, the Frame will remove the `[aria-busy]` attribute. When navigating the `<turbo-frame>` through a `<form>` submission, Turbo will toggle the `[aria-busy="true"]` attribute in tandem with the Frame's.
 
+After navigation finishes, a Frame will set the `[complete]` attribute on the
+`<turbo-frame>` element.
+
 [aria-busy]: https://www.w3.org/TR/wai-aria/#aria-busy
 
 ## Lazy-Loading Frames

_source/reference/frames.md

@@ -114,6 +114,9 @@ attributes and JavaScript properties.
 * `target` refers to another `<turbo-frame>` element by ID to be navigated when
   a descendant `<a>` is clicked. When `target="_top"`, navigate the window.
 
+* `complete` is a boolean attribute whose presence or absence indicates whether
+  or not the `<turbo-frame>` element has finished navigating.
+
 * `autoscroll` is a [boolean attribute][] that controls whether or not to scroll
   a `<turbo-frame>` element (and its descendant `<turbo-frame>` elements) into
   view when after loading. Control the scroll's vertical alignment by setting the