ARROW-11692: [Rust][DataFusion] Improve OptimizerRule comments

[alamb]

I am messing around with OptimizerRule for some experiments, and found myself clarifying the comments each time... So I figured I would at least get the doc changes queued for inclusion

[github-actions]

https://issues.apache.org/jira/browse/ARROW-11692

[Dandandan]

LGTM 👍

[codecov-io]

Codecov Report

Merging #9529 (09b3027) into master (bca7d2f) will increase coverage by 0.03%.
The diff coverage is 91.84%.

@@            Coverage Diff             @@
##           master    #9529      +/-   ##
==========================================
+ Coverage   82.24%   82.27%   +0.03%     
==========================================
  Files         244      244              
  Lines       55289    55371      +82     
==========================================
+ Hits        45470    45555      +85     
+ Misses       9819     9816       -3     

Impacted Files	Coverage Δ
rust/arrow/src/compute/kernels/comparison.rs	95.84% <ø> (ø)
rust/arrow/src/json/reader.rs	83.63% <89.89%> (+0.81%)	⬆️
rust/arrow/src/compute/kernels/cast.rs	97.28% <100.00%> (+0.04%)	⬆️
rust/datafusion/src/physical_plan/limit.rs	57.47% <100.00%> (-0.49%)	⬇️
rust/datafusion/src/physical_plan/planner.rs	79.58% <100.00%> (-0.05%)	⬇️
rust/datafusion/src/physical_plan/sort.rs	90.17% <100.00%> (+0.90%)	⬆️

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