chore: add some comments / move some code

[xxchan]

picked from #10910

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What's changed and what's your intention?

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
• All checks passed in ./risedev check (or alias, ./risedev c)
• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

• My PR contains critical fixes that are necessary to be merged into the latest release. (Please check out the details)

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Types of user-facing changes

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• Installation and deployment
• Connector (sources & sinks)
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• RisingWave cluster configuration changes
• Other (please specify in the release note below)

Release note

[xxchan]

weird 👀 cc @kwannoel

[kwannoel]

weird 👀 cc @kwannoel

Could be #10913?

Saw some failure in Dylan's recent PR also and this was the cause. Can just rerun

[Gun9niR]

LGTM on the system param part. Thanks!

[wangrunji0408]

LGTM on the UDF part.

[xxchan]

😇 again @kwannoel

[xxchan]

🤔 wrong result in e2e-parallel streaming/tpch_snapshot

https://buildkite.com/risingwavelabs/pull-request/builds/27243#01894a31-63c6-4fa0-9575-882d0883a89e

[codecov]

Codecov Report

Merging #10912 (6c74937) into main (dc6f647) will increase coverage by 0.00%.
The diff coverage is 11.23%.

@@           Coverage Diff           @@
##             main   #10912   +/-   ##
=======================================
  Coverage   69.96%   69.97%           
=======================================
  Files        1307     1307           
  Lines      223292   223292           
=======================================
+ Hits       156237   156241    +4     
+ Misses      67055    67051    -4     

Flag	Coverage Δ
rust	69.97% <11.23%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/udf/src/external.rs	0.00% <0.00%> (ø)
src/common/src/system_param/mod.rs	85.81% <100.00%> (ø)

... and 4 files with indirect coverage changes

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