Add documentation for element query operators

[Fashander]

Description

Closes #1314.

Readiness checklist

• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• I ensured that the title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[vercel]

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Name	Status	Preview	Updated
ferret-db	✅ Ready (Inspect)	Visit Preview	Dec 22, 2022 at 0:01AM (UTC)

[codecov]

Codecov Report

Merging #1675 (912346c) into main (495f54f) will decrease coverage by 0.14%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1675      +/-   ##
==========================================
- Coverage   68.96%   68.82%   -0.15%     
==========================================
  Files         290      290              
  Lines       14110    14110              
==========================================
- Hits         9731     9711      -20     
- Misses       3427     3450      +23     
+ Partials      952      949       -3     

Impacted Files	Coverage Δ
internal/handlers/tigris/tigrisdb/errors.go	21.42% <0.00%> (-21.43%)	⬇️
internal/handlers/tigris/tigrisdb/databases.go	57.50% <0.00%> (-10.00%)	⬇️
internal/handlers/tigris/msg_create.go	67.60% <0.00%> (-7.05%)	⬇️
internal/handlers/tigris/tigrisdb/collections.go	61.90% <0.00%> (-4.77%)	⬇️
internal/util/version/version.go	70.58% <0.00%> (-4.42%)	⬇️
internal/handlers/common/filter.go	84.86% <0.00%> (ø)

Flag	Coverage Δ
integration	64.68% <ø> (-0.15%)	⬇️
mongodb	14.37% <ø> (-0.01%)	⬇️
pg	53.90% <ø> (-0.03%)	⬇️
tigris	37.91% <ø> (-0.15%)	⬇️
unit	28.02% <ø> (-0.03%)	⬇️

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