docs: Add warning about async callback for describe (#5046)

* docs: Add warning about async callback for describe

I couldn't find any reference to `describe` not supporting an async function. It seems like a natural idea given `it` and `before` do.

I spent considerable time trying to figure out why an async test was failing before I discovered the reason deep in an issue discussion (#2975 (comment)), so hope that this helps others design their test suites!

* Update docs/index.md

Co-authored-by: Josh Goldberg ✨ <git@joshuakgoldberg.com>

* Update docs/index.md

Co-authored-by: Josh Goldberg ✨ <git@joshuakgoldberg.com>

---------

Co-authored-by: Josh Goldberg ✨ <git@joshuakgoldberg.com>

docs/index.md

@@ -298,6 +298,11 @@ describe('#find()', function () {
 });
 ```
 
+### Limitations of asynchronous callbacks
+
+You can use all asynchronous callbacks (`done`, `Promise`, and `async`/`await`) in callbacks for `it()`, `before()`, `after()`, `beforeEach()`, `afterEach()`) but not `describe()` -- it must be synchronous.
+See [#5046](https://github.com/mochajs/mocha/pull/5046) for more information.
+
 ## Synchronous Code
 
 When testing synchronous code, omit the callback and Mocha will automatically continue on to the next test.