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 (mochajs#2975 (comment)), so hope that this helps others design their test suites!

docs/index.md

@@ -296,6 +296,10 @@ describe('#find()', function () {
 });
 ```
 
+### Limitations of asynchronous callbacks
+
+You can use all three asynchronous patterns (`done`, `Promise`, and `async`/`await`) in callbacks for `it()` and for all hooks (`before()`, `after()`, `beforeEach()`, `afterEach()`), but `describe()` will not work correctly with an asynchronous callback -- it must be synchronous. This is because, in the "parsing" cycle all `describe` callbacks are excecuted and the test hierarchy (`runner`) is created, before any tests are run.
+
 ## Synchronous Code
 
 When testing synchronous code, omit the callback and Mocha will automatically continue on to the next test.