Add docs/changelog for EventTarget/AbortSignal improvements

[jasnell]

workerd PR: cloudflare/workerd#7053

Summary

see workerd PR: cloudflare/workerd#7053

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.