Provide a more precise definition for get_completion_scheduler than present in the user-facing design section

[LeeHowes]

Give precise definition. Maybe:
if the sender completes with set_value, it will call set_value on the returned scheduler.

[griwes]

The precise definition will go into the wording, and possibly into the implementer-centric design section somewhere. The user-side design section does not talk about receivers in the first place, so it cannot mention set_value.

[griwes]

Rename has been done, updating the title to track the remainder of this issue.

[griwes]

The specification now has wording that states that if this is not true, the behavior is undefined.