Add TODOs and fix URLs

[AlekSi]

Description

Tiny changes.

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• 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), Milestone (Next), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #3723 (3b0e91b) into main (3ed16c6) will decrease coverage by 0.46%.
The diff coverage is 75.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3723      +/-   ##
==========================================
- Coverage   31.90%   31.44%   -0.46%     
==========================================
  Files         323      323              
  Lines       23063    23069       +6     
==========================================
- Hits         7359     7255     -104     
- Misses      15027    15141     +114     
+ Partials      677      673       -4     

Files	Coverage Δ
internal/backends/postgresql/collection.go	43.67% <100.00%> (+0.27%)	⬆️
internal/backends/sqlite/collection.go	51.05% <100.00%> (+0.25%)	⬆️
internal/handlers/sqlite/msg_insert.go	0.00% <0.00%> (ø)

... and 8 files with indirect coverage changes

Flag	Coverage Δ
filter-true	5.45% <0.00%> (-0.79%)	⬇️
hana-1	?
integration	5.45% <0.00%> (-0.79%)	⬇️
mongodb-1	5.45% <0.00%> (-0.01%)	⬇️
sort-false	5.45% <0.00%> (-0.79%)	⬇️
unit	30.17% <75.00%> (+0.01%)	⬆️

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