Add TODOs for capped collections

[AlekSi]

Description

Describe future work.

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.

[mergify]

@AlekSi this pull request has merge conflicts.

[codecov]

Codecov Report

Merging #3461 (109cb4a) into main (e5e2485) will decrease coverage by 0.54%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##             main    #3461      +/-   ##
==========================================
- Coverage   74.31%   73.78%   -0.54%     
==========================================
  Files         426      426              
  Lines       26858    26875      +17     
==========================================
- Hits        19960    19830     -130     
- Misses       5738     5861     +123     
- Partials     1160     1184      +24     

Files	Coverage Δ
internal/backends/collection.go	91.66% <100.00%> (+0.41%)	⬆️
internal/backends/sqlite/collection.go	77.77% <100.00%> (-0.93%)	⬇️
internal/handlers/common/find.go	100.00% <ø> (ø)
internal/handlers/pg/msg_serverstatus.go	72.72% <100.00%> (ø)
internal/handlers/sqlite/msg_collstats.go	66.19% <100.00%> (+0.48%)	⬆️
internal/handlers/sqlite/msg_create.go	71.62% <100.00%> (ø)
internal/backends/postgresql/collection.go	14.97% <0.00%> (-0.15%)	⬇️
internal/backends/database.go	94.20% <50.00%> (-5.80%)	⬇️

... and 19 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	67.94% <70.00%> (-0.60%)	⬇️
hana-1	?
integration	67.94% <70.00%> (-0.64%)	⬇️
mongodb-1	4.57% <0.00%> (-0.01%)	⬇️
pg-1	39.29% <5.00%> (-0.50%)	⬇️
pg-2	42.36% <5.00%> (-0.57%)	⬇️
pg-3	42.86% <5.00%> (-0.65%)	⬇️
postgresql-1	∅ <ø> (∅)
postgresql-2	∅ <ø> (∅)
postgresql-3	∅ <ø> (∅)
sort-false	67.94% <70.00%> (-0.23%)	⬇️
sort-true	?
sqlite-1	38.42% <65.00%> (-0.08%)	⬇️
sqlite-2	41.69% <65.00%> (+0.03%)	⬆️
sqlite-3	42.43% <65.00%> (-0.02%)	⬇️
unit	25.60% <30.00%> (-0.03%)	⬇️

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

[chilagrow]

Looks great, just some small detail I found out while playing with capped collection 🤗