Update docs for the dummy handler

[AlekSi]

Description

Make it a bit easier to start a new handler.

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	Oct 18, 2022 at 0:50AM (UTC)

[codecov]

Codecov Report

Merging #1276 (aea1376) into main (8a9a7af) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1276      +/-   ##
==========================================
- Coverage   67.10%   67.03%   -0.08%     
==========================================
  Files         270      270              
  Lines       12721    12719       -2     
==========================================
- Hits         8537     8526      -11     
- Misses       3277     3286       +9     
  Partials      907      907              

Impacted Files	Coverage Δ
internal/handlers/dummy/dummy.go	0.00% <ø> (ø)
internal/handlers/tigris/msg_collmod.go	0.00% <0.00%> (ø)
internal/handlers/tigris/msg_explain.go	0.00% <0.00%> (ø)
internal/handlers/tigris/tigris.go	86.95% <ø> (+6.95%)	⬆️
internal/handlers/pg/pgdb/settings.go	39.53% <0.00%> (-4.66%)	⬇️
internal/util/version/version.go	48.52% <0.00%> (-4.42%)	⬇️
internal/types/compare_order.go	80.95% <0.00%> (-2.39%)	⬇️
internal/handlers/common/filter.go	86.58% <0.00%> (ø)

Flag	Coverage Δ
integration	62.94% <0.00%> (-0.08%)	⬇️
mongodb	14.08% <0.00%> (-0.04%)	⬇️
pg	52.35% <0.00%> (-0.08%)	⬇️
tigris	34.23% <0.00%> (-0.02%)	⬇️
unit	25.05% <0.00%> (-0.03%)	⬇️

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[rumyantseva]

LGTM