[manifest] help string to indicate / is the only path allowed

[arthurlutz]

Problem

• Description of why you made this PR

Solution

• And how do you fix that problem

PR Status

• [ ] Code finished.
• [ ] Tested with Package_check.
• [ ] Fix or enhancement tested.
• [ ] Upgrade from last version tested.
• [ ] Can be reviewed and tested.

Package_check results

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