Improve definitions for aggregation stages

[Fashander]

Description

Closes #3428

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 #3499 (aa5a79e) into main (e5e2485) will decrease coverage by 0.36%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3499      +/-   ##
==========================================
- Coverage   74.31%   73.96%   -0.36%     
==========================================
  Files         426      426              
  Lines       26858    26858              
==========================================
- Hits        19960    19865      -95     
- Misses       5738     5815      +77     
- Partials     1160     1178      +18     

see 11 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	68.10% <ø> (-0.43%)	⬇️
hana-1	2.93% <ø> (ø)
integration	68.10% <ø> (-0.48%)	⬇️
mongodb-1	4.57% <ø> (ø)
pg-1	39.32% <ø> (-0.47%)	⬇️
pg-2	42.49% <ø> (-0.45%)	⬇️
pg-3	42.90% <ø> (-0.62%)	⬇️
postgresql-1	∅ <ø> (∅)
postgresql-2	∅ <ø> (∅)
postgresql-3	∅ <ø> (∅)
sort-false	68.10% <ø> (-0.07%)	⬇️
sort-true	?
sqlite-1	38.40% <ø> (-0.10%)	⬇️
sqlite-2	41.55% <ø> (-0.11%)	⬇️
sqlite-3	42.42% <ø> (-0.03%)	⬇️
unit	25.59% <ø> (-0.04%)	⬇️

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