Document eager futures

[Amir Kamil]

Impl PR 345 is close to merging, and it implements eager futures as described in the working group draft in spec PR 41. These changes need to be documented in the spec proper. The WGD in PR 41 contains language that should be importable into the spec, but we’ll also need to audit the spec for any assumptions about asynchronous completion.

Note that impl PR 345 does not change the semantics of operation_cx::as_future() – for now, it will be equivalent to operation_cx::as_defer_future(). However, the user can define UPCXX_DEFER_COMPLETION to 0 to make it equivalent to operation_cx::as_eager_future() instead. Should we change the default, we will need to make further changes to the spec to reflect that. In either case, we will need to decide whether UPCXX_DEFER_COMPLETION should be documented in the spec proper or as an implementation-defined feature.

Relevant spec issues:

• issue 107
• issue 28

‌

[Dan Bonachea]

Documentation tasks here also include updating the Impl ChangeLog to reflect the final/release state of this feature

[Amir Kamil]

Spec documentation done in PR 71. Implementation documentation tasks moved over to impl issue 486.