Clarify me.org output config docs

[Mirochill]

Summary:

• document that meorg_output_name is a per-realisation boolean at the same level as repo
• add a warning that only one realisation can enable the me.org output workflow
• fix the YAML example indentation and clarify when name should be set explicitly

Related issues:

• Closes Docs for model_output_name need fixing #343

Validation:

• rg -n "meorg_output_name|More than 1 value set|model output name" docs/user_guide/config_options.md src/benchcab/config.py src/benchcab/data/config-schema.yml
• git diff --check
• git diff --cached --check
• git show --check --stat --oneline HEAD
• Not run locally: docs build or test suite