docs: clarify job callback contract

[ccarvalho-eng]

Summary

• document the metadata map passed to job callbacks in the README
• clarify that snoozed jobs are rescheduled with an explicit delay and still count in retry accounting
• align the job and worker moduledocs with the current snooze behavior

[ccarvalho-eng]

Open question on the snooze wording: current implementation appears to count {:snooze, delay_ms} in retry accounting because it flows through Store.requeue/4 and increments error_count, and the existing integration test asserts that behavior.

This PR updates the docs to match that current behavior. Just want to confirm that is intentional before we lock the wording in; if snooze is meant to be a pure delay without consuming retry budget, I can change this PR to leave the docs alone and open a behavior PR instead.