Improve embedded package documentation

[princejha95]

Description

Closes #2326.

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 #3537 (1227b26) into main (d1486f2) will decrease coverage by 0.66%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3537      +/-   ##
==========================================
- Coverage   75.55%   74.89%   -0.66%     
==========================================
  Files         427      427              
  Lines       27131    27131              
==========================================
- Hits        20499    20320     -179     
- Misses       5448     5600     +152     
- Partials     1184     1211      +27     

Files	Coverage Δ
ferretdb/ferretdb.go	82.65% <ø> (ø)

... and 18 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	71.72% <ø> (-0.77%)	⬇️
hana-1	?
integration	71.72% <ø> (-0.81%)	⬇️
mongodb-1	4.52% <ø> (ø)
pg-1	42.49% <ø> (-0.56%)	⬇️
pg-2	39.93% <ø> (-0.49%)	⬇️
pg-3	41.62% <ø> (-0.51%)	⬇️
postgresql-1	41.69% <ø> (-0.04%)	⬇️
postgresql-2	39.14% <ø> (-0.10%)	⬇️
postgresql-3	41.41% <ø> (-0.15%)	⬇️
sort-false	71.72% <ø> (-0.25%)	⬇️
sort-true	?
sqlite-1	41.87% <ø> (-0.06%)	⬇️
sqlite-2	39.13% <ø> (-0.03%)	⬇️
sqlite-3	40.97% <ø> (-0.02%)	⬇️
unit	25.91% <ø> (-0.08%)	⬇️

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