MINOR: [Doc][Python] Clarify behavior of pyarrow.compute.fill_value

[spenczar]

Discussed in #35624, particularly at #35624 (comment).

Doesn't close that issue, but it came up in discussion.

[pitrou]

Thanks @spenczar ! Just one nit.

[spenczar]

Oops! Thanks for directly fixing it :)

[pitrou]

CI failures are unrelated.

[ursabot]

Benchmark runs are scheduled for baseline = 2695293 and contender = e628ca5. e628ca5 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.73% ⬆️0.12%] test-mac-arm
[Finished ⬇️2.29% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️1.08% ⬆️0.12%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] e628ca50 ec2-t3-xlarge-us-east-2
[Failed] e628ca50 test-mac-arm
[Finished] e628ca50 ursa-i9-9960x
[Finished] e628ca50 ursa-thinkcentre-m75q
[Finished] 26952930 ec2-t3-xlarge-us-east-2
[Failed] 26952930 test-mac-arm
[Finished] 26952930 ursa-i9-9960x
[Finished] 26952930 ursa-thinkcentre-m75q
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