Minor: Add docstrings to EliminateOuterJoins optimizer pass

[alamb]

Which issue does this PR close?

re #4272

Rationale for this change

I noticed in #4272 from @jackwener that some comments were lost. I found this kind of rewrite somewhat mind bending, so figured I would put them back

What changes are included in this PR?

Add docstrings to EliminateOuterJoins

Are these changes tested?

No

Are there any user-facing changes?

Not really

[ursabot]

Benchmark runs are scheduled for baseline = 561be4f and contender = e1204a5. e1204a5 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