Changed ruleset for Enum types.

[thempen]

The ruleset for enum definition "shall" is not useful in many cases where "UNKNOWN" and "OTHER" is more confusing than helpful. So it should only be a recommendation for future implmentations.

Reference to a related issue in the repository

There are already multiple enums without the UNKNOWN and OTHER definition, like in https://github.com/OpenSimulationInterface/open-simulation-interface/blob/master/osi_motionrequest.proto. There are also current PRs where the current definition is appropriate.

Add a description

The rule to add "UNKNOWN" and "OTHER" should only be a recommendation in the future.

Mention a member

@thempen

Check the checklist

• [x] I have performed a self-review of my own code/documentation.
• [x] My documentation changes are related to another repository in the organization. Here is the link to the issue/repo.
• [x] My changes generate no new warnings during the documentation generation.
• [x] The existing travis ci which pushes the documentation to gh-pages passes with my changes.

[pmai]

CCB 2024-03-11: Merge as-is.