Updates to the HTMLDialogElement.returnValue example

[wbamberg]

This makes some updates to the example for https://developer.mozilla.org/en-US/docs/Web/API/HTMLDialogElement/returnValue.

It's mostly motivated by some of the issues discussed in #39336. I also made a few other changes though:

• updated the description for clarity
• separated HTML and JavaScript
• added subheadings
• some minor code updates, like using the ternary operator and template strings, and more consistent naming.

[github-actions]

Preview URLs

• /en-US/docs/Web/API/HTMLDialogElement/returnValue

Flaws (2)

URL: /en-US/docs/Web/API/HTMLDialogElement/returnValue
Title: HTMLDialogElement: returnValue property
Flaw count: 2

• macros:
  • Macro produces link /en-US/docs/Web/HTML/Element/dialog which is a redirect
  • Macro produces link /en-US/docs/Web/HTML/Element/dialog which is a redirect