Document `--create-origin` on the swh-deposit client for origin creation

In #2377 (closed) we identified that a deposit via the deposit client with the paramslug resulted with an generated uuid in its origin. This behavior should be documented in the client documentation and in the code.

There are two scenarios with the deposit:

• a deposit with a publicly available location that can be identified as the origin
• a deposit without a publicly available location

mention deprecation of the --slug option

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Migrated from T2391 (view on Phabricator)