docs: Document that get_starting_timestamp requires setting a non-null replication_key

[MeltyBot]

Migrated from GitLab: https://gitlab.com/meltano/sdk/-/issues/322

Originally created by @edgarrmondragon on 2022-01-31 19:53:18

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From slack thread: https://meltano.slack.com/archives/C01TCRBBJD7/p1643646009947879

Stream.get_starting_timestamp(context) returns None if Stream.replication_key is not set for the stream. Documentation for get_starting_timestamp(context) should indicate that requirement.

[MeltyBot]

View 1 previous comment from the original issue on GitLab

[tayloramurphy]

@edgarrmondragon is this still relevant?

[edgarrmondragon]

@edgarrmondragon is this still relevant?

@tayloramurphy yeah, still relevant. We do have https://sdk.meltano.com/en/latest/incremental_replication.html now, but it wouldn't hurt to be explicit.

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[edgarrmondragon]

Still relevant

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