Fix description of syslog.severity.name and syslog.severity.code

[fgierlinger]

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• Have you added an entry to the CHANGELOG.next.md? Yes

This PR corrects the wording of the log.syslog.severity.name description, to specify that this field should contain a text-based severity. This was already mentioned in the short description.

Closes #1948

[github-actions]

Documentation changes preview: https://ecs_2290.docs-preview.app.elstc.co/diff

[ebeahan]

@elasticmachine run elasticsearch-ci/docs

[github-actions]

This PR is stale because it has been open for 60 days with no activity.