[uss_qualifier] adapt SCD subscription sync documentation to same convention as OIR

[Shastick]

While iterating on #535 we ended on conventions that differ from what was done for subscriptions.

This aligns the existing subscription synchronization scenario with what has been/is being done for operational intents.

Mainly:

• format and content checks are properly separated
• format checks now have a high severity, as they might imply that we can't verify the content
• content checks now have a medium severity, as failing them does not preclude continuation of the scenario