ARROW-14203: [C++] Fix description of ExecBatch.length for Scalars in aggregate kernels
ARROW-14203: [C++] Fix description of ExecBatch.length for Scalars in aggregate kernels

This PR updates the comment describing the `length` data member of ExecBatch to clarify its possible values when operating on Scalars in (non-)aggregate kernels.

Closes #11416 from edponce/ARROW-14203-Fix-description-of-ExecBatch-length-for-

Authored-by: Eduardo Ponce <6a4c8f271abaad6949809c569bf8d3844179d012@gmail.com>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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