[docs] Improvements to RMT docs

[gbuenodevsuse]

Description

Small changes to docs to improve readability and cover more topics such as tests.

Change Type

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• Bug Fix (a non-breaking change which fixes an issue)
• New Feature (a non-breaking change which adds new functionality)
• Documentation Update (a change which only updates documentation)

Checklist

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• I have reviewed my own code and believe that it's ready for an external review.
• I have provided comments for any hard-to-understand code.
• I have documented the MANUAL.md file with any changes to the user experience.
• If my changes are non-trivial, I have added a changelog entry to notify users at package/obs/rmt-server.changes.

Review

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[digitaltom]

I think this can be understood in a misleading way, because RMT is not a systems management solution. We could point out to use MLM if that is needed.

[digitaltom]

Thanks for you contribution! Please check the comment about 'system management'.