Update cluster.rst - added generic thread pool definition

[karynzv]

Summary of the changes / Why this improves CrateDB

Added the description of generic thread pool to the thread pools subsection

Checklist

• Added an entry in the latest docs/appendices/release-notes/<x.y.0>.rst for user facing changes
• Updated documentation & sql_features table for user facing changes
• Touched code is covered by tests
• CLA is signed
• This does not contain breaking changes, or if it does:
  • It is released within a major release
  • It is recorded in the latest docs/appendices/release-notes/<x.y.0>.rst
  • It was marked as deprecated in an earlier release if possible
  • You've thought about the consequences and other components are adapted
    (E.g. AdminUI)

[mfussenegger]

Thanks for the PR. I changed the generic definition slightly, changed the target branch to master (please always make PRs against master, unless the change is really only specific to a older release and it's already fixed in master), and also added another commit to elaborate more on when tuning the size might make sense and when it doesn't

@romseygeek @matriv and @seut please have another look.

[mfussenegger]

Looks like vale failure is a false positive (detecting to touch as to to repetition), doesn't happen with my locale vale so I'll look into updating the vale version on CI

[mfussenegger]

I'll look into updating the vale version on CI

Actually, would be nice if somebody could take this over. Otherwise this will have to wait until next week

[matriv]

I've amended the second commit with some minor changes (like adding link for refresh), some commas, etc. @seut Can you please take a look before we merge it?

Update: also updated vale in a new commit, seems to work now.

[seut]

👍 thanks!

[matriv]

@Mergifyio backport 5.6

[mergify]

backport 5.6

✅ Backports have been created

• #15701 Update cluster.rst - added generic thread pool definition (backport #15690) has been created for branch 5.6