docs: clarify invalid usage of done() (#4101)

When I first read 
"anything else will cause a failed test"
I considered that this might be a typo,
and that this should say "a passed test"
instead, because after all setting "no error"
could mean "no error", right? I then understood
that this sentence could also mean that
"anything else is a programmer error", i.e. a bug
and I was hoping that in _that_ case a clear
error would be thrown.

I tried things out and indeed found that the latter
is the case

> Error: done() invoked with non-Error: true

This patch attempts to clarify that this is
indeed all about a programmer error.

docs/index.md

@@ -234,7 +234,7 @@ Mocha allows you to use any assertion library you wish. In the above example, we
 
 ## Asynchronous Code
 
-Testing asynchronous code with Mocha could not be simpler! Simply invoke the callback when your test is complete. By adding a callback (usually named `done`) to `it()`, Mocha will know that it should wait for this function to be called to complete the test. This callback accepts both an `Error` instance (or subclass thereof) _or_ a falsy value; anything else will cause a failed test.
+Testing asynchronous code with Mocha could not be simpler! Simply invoke the callback when your test is complete. By adding a callback (usually named `done`) to `it()`, Mocha will know that it should wait for this function to be called to complete the test. This callback accepts both an `Error` instance (or subclass thereof) _or_ a falsy value; anything else is invalid usage and throws an error (usually causing a failed test).
 
 ```js
 describe('User', function() {