Document promise extensions

[Dan Bonachea]

This issue exists to ensure we update the spec wrt the outcome of the changes being discussed in implementation issue 277 and pull request #141.

Currently, this includes at least:

1. Explaining that promise is a handle to a reference-counted object, not an unboxed type (and maybe mention this relaxation was added in 2019.12.0)
2. Adding C++ Concept CopyAssignable to promise
3. Changing completion_cx::as_promise to take the promise argument by-value
4. Changing all the promise mutator methods to be const-qualified

[Dan Bonachea]

Implementation pull request #141 has been merged.

Documenting the new functionality is now a blocker for the March spec.

[Dan Bonachea]

proposed resolution in pull request #34

[Amir Kamil]

Fix #156 - document promise extensions.

→ <<cset 6d27c043546c>>

[Amir Kamil]

Merged in akamil/upcxx-spec/issue156 (pull request #34)

Fix #156 - document promise extensions.

Approved-by: Dan Bonachea [e-mail] Approved-by: Paul Hargrove [e-mail] Approved-by: John Bachan [e-mail]

→ <<cset f53153a0eec4>>