Do various small cleanups

[AlekSi]

Description

Refs #2236.

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #2561 (5c38065) into main (30dd842) will increase coverage by 21.93%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##            main    #2561       +/-   ##
==========================================
+ Coverage   5.04%   26.97%   +21.93%     
==========================================
  Files        403      405        +2     
  Lines      19838    20086      +248     
==========================================
+ Hits        1000     5419     +4419     
+ Misses     18763    14095     -4668     
- Partials      75      572      +497     

Impacted Files	Coverage Δ
internal/handlers/common/params.go	3.92% <0.00%> (+3.92%)	⬆️
internal/types/document.go	72.59% <ø> (+39.90%)	⬆️
internal/util/iterator/count.go	0.00% <ø> (ø)
internal/util/iterator/values.go	94.73% <ø> (+94.73%)	⬆️
internal/util/must/must.go	28.57% <0.00%> (-11.43%)	⬇️

... and 131 files with indirect coverage changes

Flag	Coverage Δ
integration	5.03% <0.00%> (-0.01%)	⬇️
mongodb	5.03% <0.00%> (-0.01%)	⬇️
unit	25.60% <0.00%> (?)

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[w84thesun]

👍