Clarify visits in SWHIDs

The current SWHID specification mentions:

* **visit** : the *status of a full repository* containing the designated object, as a *snapshot*
  corresponding to a specific *visit* of that repository;

I find it confusing to call it visit if it's a snapshot PID. If it's because of the lack of visit identifiers, maybe we should fix that issue instead.

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Migrated from T2387 (view on Phabricator)