K8s tutorial: Clarify how status is set if no relation data

[dwilding]

In Integrate your charm with PostgreSQL, the definition of fetch_postgres_relation_data has a comment that says:

[...] If no data is retrieved, the unit is set to waiting status and the program exits with a zero status code.

This is confusing for several reasons:

• The method itself doesn't set the unit status. Instead, later in the doc we're adding a collect-status handler that sets waiting status.
• Even in the collect-status handler, fetch_postgres_relation_data isn't called. Instead, both places use self.database.fetch_relation_data.
• "The program exists with a zero status" is technically true but not very helpful. We don't usually think of charms as programs that exit with particular status codes.

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Discussion in Matrix: https://matrix.to/#/!LhFxJIPEcCacgdMghH:ubuntu.com/$yreC1AD_wDNvYYL18CyzXL6UPOp1yeCbVV28BpiIswc