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MINOR: [R] Fix default value and docs of write_dataset

The default value was `NA`, which is different from the `0L` in the description, so it is corrected to `0L`.
And, the explanation that it is unlimited when set to `0L` was missing, so it was added.

Closes #12418 from eitsupi/fix-dataset-write

Authored-by: SHIMA Tatsuya <ts1s1andn@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>