doc/promtail/journal: adds available labels paragraph

[arcenik]

• Adds journal available labels paragraph

What this PR does / why we need it:

Adds a paragraph promtail/journal documentation about _journal* labels.

Which issue(s) this PR fixes: Fixes #2048

Special notes for your reviewer:

Checklist

• [x] Reviewed the CONTRIBUTING.md guide (required)
• [x] Documentation added
• [x] Tests updated
• [x] CHANGELOG.md updated
  • [x] If the change is worth mentioning in the release notes, add add-to-release-notes label
• [x] Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• [x] For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• [x] If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

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[grafanabot]

This PR must be merged before a backport PR will be created.

[grafanabot]

This PR must be merged before a backport PR will be created.