Document the Buffer/Batch constraints on tower-batch

[teor2345]

Is your feature request related to a problem? Please describe.

tower-batch has a very similar implementation to tower::buffer:
https://github.com/ZcashFoundation/zebra/blob/main/tower-batch/src/service.rs#L97
https://docs.rs/tower/0.4.4/src/tower/buffer/service.rs.html#112

Like tower::buffer:

• If lots of tasks with data dependencies use a Batch with a small bound, the Batch can hang.
• Batches with small bounds can also limit concurrency, particularly if the underlying service does a lot of work in its call.

Describe the solution you'd like

Document these risks, but be more detailed than tower::buffer:
https://docs.rs/tower/0.4.4/tower/buffer/struct.Buffer.html#a-note-on-choosing-a-bound

We probably want to add the details at module level, then add a summary on new and poll_ready:
https://github.com/ZcashFoundation/zebra/blob/main/tower-batch/src/lib.rs
https://github.com/ZcashFoundation/zebra/blob/main/tower-batch/src/service.rs#L53
https://github.com/ZcashFoundation/zebra/blob/main/tower-batch/src/service.rs#L87

Describe alternatives you've considered

Do nothing. Callers risk hangs in tower-buffer.

Additional context

See #1593 for the overall Buffer hang issue, analysis, and design patterns.

[teor2345]

This task has been revised based on the fixes to the underlying Buffer issue, see #1593 for details.

[teor2345]

The current settings seem fine.