docs: document pruneFilters, currentContext, cliPluginsExtraDirs

[GitNimay]

• Related to: Complete documentation on CLI configuration file #1086

Summary

Documents three config.json properties that were missing from the configuration file reference:

• pruneFilters: default filters for the prune commands. They're combined with --filter, and conflicting label/label! filters from the config file are ignored (see command.PruneFilters).
• currentContext: the default context, set by docker context use. --context, --host, DOCKER_CONTEXT and DOCKER_HOST take precedence (see resolveContextName).
• cliPluginsExtraDirs: extra directories to search for CLI plugins. They're searched first, so they take precedence (see getPluginDirs).

I also added them to the sample configuration file.

The orchestrator and kubernetes options mentioned in the issue no longer exist, so this PR doesn't close it.

Release notes (optional)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[GitNimay]

@thaJeztah @vvoland PTAL when you have a moment. The only failing check is validate-milestone, which needs a milestone set on the PR (expected: 29.10.0). All other checks pass. This is a docs-only change, so the changelog entry is left empty.