docs: Add warning about async callback for describe

[mcdurdin]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions.

Description of the Change

This is a documentation only change.

I couldn't find any reference to describe not supporting an async callback. It seems like a natural conclusion to draw, given mocha specializes in async tests, and given it and hook do.

I spent considerable time trying to figure out why an async test was failing in one of my modules before I discovered the reason deep in an issue discussion (#2975 (comment))!

Alternate Designs

Why should this be in core?

This is a documentation change.

Benefits

I hope that this helps others design their test suites (I saw a number of comments about spending 'lots of time' trying to figure this out).

Possible Drawbacks

Applicable issues

• Describe block with async function behaving weirdly #2975
• 🚀 Feature: Allow async function in describe (block) #2116
• Please allow describe can do async with done/promise #2085

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: mcdurdin / name: Marc Durdin (65133eb, 9e708d9, f2d6fca)

[JoshuaKGoldberg]

LGTM! Just a couple small touchups for typos.

[JoshuaKGoldberg]

Note that #4921 should also help with this.

[voxpelli]

Can we simplify it a bit?

[JoshuaKGoldberg]

👋 @mcdurdin is this still something you have time for? Just checking in!

[JoshuaKGoldberg]

I'll go ahead and take the liberty of merging this now, as it's a docs change. Thanks for the productive conversation @mcdurdin! 🙌