Using promises: add Cancellation section (and link to AbortController)

[fowl2]

Description

Adding a very simple section describing cancellation.

Motivation

Because how do you cancel a promise?

related: #33346

I'm not sure if there is a more JavaScripty term than "cancellation", but we probably don't want the noun of "abort".

[github-actions[bot]]

Preview URLs

• /en-US/docs/Web/JavaScript/Guide/Using_promises