Fix initiallyopen ordering text

This PR fixes the text around the ordering of `initiallyopen`,
stating that only the **first** popup will be shown, rather than
the last. See [this comment](openui#494 (comment)).

research/src/pages/popup/popup.research.explainer.mdx

@@ -130,7 +130,7 @@ As mentioned above, a `<div popup=popup>` will be hidden by default. If the popu
 <div popup=popup initiallyopen>
 ```
 
-In this case, the UA will immediately call `showPopup()` on the element, as it is parsed. If multiple such elements exist on the page, each subsequent popup will "one-at-a-time" close the prior popups, leaving only the last one showing in the end.
+In this case, the UA will immediately call `showPopup()` on the element, as it is parsed. If multiple such elements exist on the page, only the first such element (in DOM order) will be shown.
 
 
 ## Non-"shown" popups
@@ -253,11 +253,6 @@ A new attribute, `anchor`, should be added for all elements, whose value is an i
 2. The referenced anchor element could be used by the [Anchor Positioning feature](https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/CSSAnchoredPositioning/explainer.md).
 
 
-## Display Ordering
-
-It is possible for an HTML document to contain multiple elements with `popup=popup`. In this case, only the last such element on the page will remain open when the page is fully loaded. This is because each successive popup element will force the previous one closed via the “one at a time” behavior.
-
-
 ## Accessibility / Semantics
 
 Since the `popup` content attribute can be applied to any element, and only impacts the element's presentation (top layer vs not top layer), this should not have **any semantic or accessibility impact**. I.e., the element with the `popup` attribute will keep its existing semantics and AOM representation. In the event an author needs to extend or modify a particular element's ARIA semantics, this may be done in accordance to existing allowances of [ARIA in HTML](https://w3c.github.io/html-aria/).