MINOR: add df.to_unoptimized_plan() to docs, remove erroneous comment

[kmitchener]

Which issue does this PR close?

Closes #.

Rationale for this change

What changes are included in this PR?

adds dataframe.to_unoptimized_plan() to docs

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #3348 (26e33b8) into master (a49e01e) will increase coverage by 0.00%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #3348 +/- ##
=======================================
Coverage 85.48% 85.48% =======================================
Files 294 294 Lines 54072 54072 =======================================
+ Hits 46223 46225 +2 + Misses 7849 7847 -2 

Impacted Files	Coverage Δ
datafusion/core/src/dataframe.rs	89.02% <ø> (ø)
datafusion/expr/src/logical_plan/plan.rs	78.22% <0.00%> (+0.16%)	⬆️
datafusion/core/src/physical_plan/metrics/value.rs	87.43% <0.00%> (+0.50%)	⬆️

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[andygrove]

Thanks @kmitchener

[ursabot]

Benchmark runs are scheduled for baseline = f4468ea and contender = 0fc7297. 0fc7297 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