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MINOR: [Documentation][C++][Python][R] Clarify docstrings around max_chunksize (#40251)

### Rationale for this change

A user [ran into confusion](https://lists.apache.org/thread/8ym8r1z5gys7dpcr8rw8dvjbkqc2lf7f) over the units of the `max_chunksize` argument in PyArrow and didn't see any reason not to make the documentation more explicit.

### What changes are included in this PR?

Just changes to docstrings.

### Are these changes tested?

No, though I did go through every change to see if it was correct and I'm pretty sure it's right. Good to double-check during review though.

### Are there any user-facing changes?

Just docs.

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>