MINOR: add some comments about intended use of ChunkedStore

[alamb]

Which issue does this PR close?

N/A

Rationale for this change

There appeared to be confusion (maybe meta confusion) about the usecase for ChunkedStream on #4525

What changes are included in this PR?

Add some docstrings (adapted from @tustvold and @metesynnada 's comments on #4525) to ChunkedStore

Are these changes tested?

Docstring only tests

Are there any user-facing changes?

No

[tustvold]

Thank you

[ursabot]

Benchmark runs are scheduled for baseline = 5f74284 and contender = 4c1f60c. 4c1f60c is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java