Clean-up some code and comments

[AlekSi]

Description

To make the next PR smaller and focused.

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 #2904 (78b4e81) into main (d2219f2) will decrease coverage by 4.99%.
The diff coverage is 87.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2904      +/-   ##
==========================================
- Coverage   63.57%   58.58%   -4.99%     
==========================================
  Files         451      451              
  Lines       23569    23570       +1     
==========================================
- Hits        14983    13808    -1175     
- Misses       7619     8766    +1147     
- Partials      967      996      +29     

Impacted Files	Coverage Δ
ferretdb/ferretdb.go	83.65% <ø> (ø)
internal/handlers/common/count_iterator.go	96.66% <ø> (ø)
...ternal/handlers/commoncommands/msg_listcommands.go	0.00% <ø> (-100.00%)	⬇️
internal/util/iterator/func.go	71.42% <ø> (ø)
internal/util/iterator/slice.go	100.00% <ø> (ø)
internal/util/iterator/values.go	84.09% <ø> (ø)
internal/util/iterator/with_close.go	0.00% <ø> (ø)
internal/util/telemetry/reporter.go	44.00% <0.00%> (ø)
integration/setup/client.go	81.25% <100.00%> (ø)
integration/setup/listener.go	62.25% <100.00%> (ø)
... and 4 more

... and 43 files with indirect coverage changes

Flag	Coverage Δ
integration	51.48% <100.00%> (-5.34%)	⬇️
mongodb	4.41% <28.57%> (+0.21%)	⬆️
pg	51.29% <100.00%> (-5.46%)	⬇️
shard-1	41.97% <100.00%> (+0.04%)	⬆️
shard-2	45.10% <100.00%> (+0.03%)	⬆️
shard-3	?
unit	24.28% <50.00%> (+<0.01%)	⬆️

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

[rumyantseva]