MINOR: Add comments about rewrite_disjunctive_predicate

[alamb]

Which issue does this PR close?

re #217 and #78

Rationale for this change

While was working on #3334, @DhamoPS noticed that TPCH Q19 was not being rewritten, despite the code in https://github.com/apache/arrow-datafusion/blob/master/datafusion/optimizer/src/rewrite_disjunctive_predicate.rs from @xudong963

However, I think it was not very clear what the intent of the rewrite pass was so I figured that some docstrings would help

What changes are included in this PR?

Add docstrings to optimizer pass, and move structure up so the comments are more discoverable

Are there any user-facing changes?

No

[alamb]

I just moved this code to the top of the module so that the comments describing what it does can be attached to the structure as well as being at the top of the file, for better discoverability

No change in behavior is intended

[xudong963]

Nice, thank you @alamb

[codecov-commenter]

Codecov Report

Merging #3351 (1b1e8e4) into master (61602f0) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3351   +/-   ##
=======================================
  Coverage   85.48%   85.48%           
=======================================
  Files         294      294           
  Lines       54120    54120           
=======================================
  Hits        46265    46265           
  Misses       7855     7855           

Impacted Files	Coverage Δ
...ion/optimizer/src/rewrite_disjunctive_predicate.rs	96.75% <100.00%> (ø)
datafusion/expr/src/logical_plan/plan.rs	77.44% <0.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[ursabot]

Benchmark runs are scheduled for baseline = 0bbf127 and contender = d7fa064. d7fa064 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