vine: fix doc typos, code typos, and callback signatures for `FuturesExecutor`

[gpauloski]

Proposed changes

Please describe your changes (e.g., what problems they attempt to solve, what results are expected, etc.) Additional motivation and context are welcome. Please also mention relevant issues and pull requests as appropriate.

This PR fixes four of the six issues discussed in #3835 (specifically, 1, 2, 4, and 5).

Post-change actions

Put an 'x' in the boxes that describe post-change actions that you have done. The more 'x' ticked, the faster your changes are accepted by maintainers.

• [x] make test Run local tests prior to pushing.
• [x] make format Format source code to comply with lint policies. Note that some lint errors can only be resolved manually (e.g., Python)
• [x] make lint Run lint on source code prior to pushing.
• [x] Manual Update Did you update the manual to reflect your changes, if appropriate? This action should be done after your changes are approved but not merged.
• [x] Type Labels Select github labels for the type of this change: bug, enhancement, etc.
• [x] Product Labels Select github labels for the product affected: TaskVine, Makeflow, etc.
• [x] PR RTM Mark your PR as ready to merge.

Additional comments

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[BarrySlyDelgado]

Thanks for noting these issues! a more detailed comment has been posted here: #3835. In short, these changes should fix that mentioned issues (1, 2, 4, 5). Issue 3 will be fixed in a forthcoming PR while 6 may require more thought regarding the implementation of the futures executor along with TaskVine.

[dthain]

@BarrySlyDelgado is this RTM?

[BarrySlyDelgado]

Yes.