minor: add some comments to row group pruning tests #4823
Merged
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Which issue does this PR close?
None
Rationale for this change
While reviewing #4810 from @liukun4515 I had to double check what this function was doing, so I wanted to document it to save future self (and maybe other readers)
What changes are included in this PR?
doc comments on a test functon
Are these changes tested?
No (nor are they needed)
Are there any user-facing changes?
No