The upgrade step of custom elements are missing in the `define` page ?

### MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/CustomElementRegistry/define#description

### What specific section or headline is this issue about?

Description

### What information was incorrect, unhelpful, or incomplete?

Step - 18, I think thats how most of the custom elements get consumed (?) , the browser upgrades the custom element , https://html.spec.whatwg.org/multipage/custom-elements.html#dom-customelementregistry-define ,
ik , this exists https://developer.mozilla.org/en-US/docs/Web/API/CustomElementRegistry/upgrade#description , but I dont really know, I feel like 1 or 2 lines about `upgrade` should be there. Eg - the constructor should be run and connectedcallback also should be run.

Can I send a PR ?

### What did you expect to see?

A few lines about the upgrade event

### Do you have any supporting links, references, or citations?

https://html.spec.whatwg.org/multipage/custom-elements.html#dom-customelementregistry-define step 18

### Do you have anything more you want to share?

_No response_

Start with the MDN CustomElementRegistry.define Description section and compare its step 18 reference with the WHATWG custom-elements specification. Review the existing CustomElementRegistry.upgrade documentation, then add a concise, accurate explanation of the relevant upgrade behavior and verify the wording against the specification.

Written by the indexing model from the issue text.