Clarify StreamExt::timeout() docs

The [description](https://docs.rs/async-std/1.6.0/async_std/prelude/trait.StreamExt.html#method.timeout) for `StreamExt::timeout()` states it will "await a stream or times out after a duration of time," but it's not clear whether it times out after the specified duration

1. since calling `timeout()`

or

2. since a stream item was last produced

I wanted #2 when I used this function, but then I realized it actually works like #1.

I noticed theres [a pull request](https://github.com/async-rs/async-std/pull/732) that adds a `timeout_repeat()` function that implements #2; the docs on both these functions should make it clear which of the above behaviors it implements.