future.then() is identical to future.unit()
future.then() is identical to future.unit()

Summary:
Document that `future.then()` is identical to `future.unit()`.

Put functions next to each other to make it clear, and use identical
wording.

Reviewed By: yfeldblum

Differential Revision: D5955451

fbshipit-source-id: 1b55d5785dc0995d4d9364f48241e98ad01b31f4

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