Clarify StreamExt::timeout() docs

[Seeker14491]

The description 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 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.