[all hosts] (Common APIs) Trimming down Common async article

[AlexJerabek]

The Asynchronous programming in Office Add-ins article has a low engagement score according to our internal metrics. This PR aims to spruce up the article in the following ways:

• Assume the reader has greater knowledge of JS programming patterns.
• Increase the number of indications this pertains to the Common APIs.
• Apply a general style check and writing refresh.

If you think I've pruned too much or changed the meaning of something, please let me know.

[learn-build-service-prod]

Learn Build status updates of commit 6353d8c:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/develop/asynchronous-programming-in-office-add-ins.md	✅Succeeded	View

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