docs: add cancel-safety docs to StreamExt::next methods and types

[guswynn]

Motivation

Many futures are commonly used in select! (or similar) constructs, and people are interested to know which futures are cancel-safe. Two such common features are next and try_next, to get new values from a Stream. These only hold onto references to the underlying stream, and are therefor cancel-safe.

Solution

Document the cancel-safety of the StreamExt::next and StreamExt::try_next methods (and their underlying future types)

[Darksonn]

Thanks.