Clarify initiator completion semantics for rput_*then_rpc

[Dan Bonachea]

Currently the API reference for rput_then_rpc seems to imply remote completion indication to the initiator (via future/promise/continuation). Despite that, the variables are named things like src_free that imply local completion.

This is inconsistent with the VIS versions of rput_*_then_rpc which claim to expose local completion to the initiator via the same mechanisms.

We should really be consistent in both places and clear about what completion is being signaled to the initiator. I think we probably want local completion everywhere, rather than forcing an acknowledgement message to establish remote completion for signalling puts (which is not typically required in apps using signalling put, and could easily be constructed by the rpc on the off-chance it's needed).

[Amir Kamil]

I'm assigning to John, as I don't know what the intended notification behavior is. I also don't really have an opinion as to which behavior is preferable.

[Amir Kamil]

OK, I've tried to make them consistent with the VIS versions, but John should do a sanity check on this.