Document cancellation safety of the async APIs.

[alilleybrinker]

We expose some async APIs but don't currently document the cancellation safety of them. This is critical for explaining what kind of state the APIs may leave things in if they are cancelled early, and what the caller should do, if anything, to handle that.

For example, the GitOid async APIs could leave the reader with partial reads. The caller should create a new reader (I think) to restart if cancelled early.