[Journald] Document the parsers

[belimawr]

The Journald input supports the same parsers as the Filestream input, here is an example:

filebeat.inputs:
  - type: journald
    id: PR-testing
    parsers:
      - multiline:
          type: count
          count_lines: 3

However this is not documented. We need to add documentation and examples both in our public documentation and configuration files.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)