Add query plan cache commands to the docs

[b1ron]

Description

Refs #1399.

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	Nov 18, 2022 at 11:36AM (UTC)

[codecov]

Codecov Report

Merging #1501 (c9a1949) into main (0d1ece0) will decrease coverage by 0.05%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1501      +/-   ##
==========================================
- Coverage   67.85%   67.80%   -0.06%     
==========================================
  Files         279      279              
  Lines       13500    13500              
==========================================
- Hits         9161     9154       -7     
- Misses       3418     3422       +4     
- Partials      921      924       +3     

Impacted Files	Coverage Δ
internal/util/version/version.go	48.52% <0.00%> (-4.42%)	⬇️
internal/handlers/common/filter.go	84.65% <0.00%> (-1.16%)	⬇️
internal/handlers/pg/pgdb/query.go	71.11% <0.00%> (+2.22%)	⬆️
internal/handlers/tigris/msg_listdatabases.go	50.70% <0.00%> (+5.63%)	⬆️

Flag	Coverage Δ
integration	63.85% <ø> (-0.09%)	⬇️
mongodb	14.17% <ø> (-0.03%)	⬇️
pg	52.99% <ø> (-0.12%)	⬇️
tigris	36.17% <ø> (+<0.01%)	⬆️
unit	26.44% <ø> (+<0.01%)	⬆️

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