Workflow docs: instance ID reuse requires the whole workflow tree be terminal

[JoshVanL]

The workflow identity docs added for the 1.18 reuse policy removal stated that creating a workflow whose instance ID already exists is rejected in any state, including completed. That never matched the runtime: recreating an instance in a terminal state has been allowed since the workflow engine was introduced, and remains so.

Document the contract as enforced from 1.18.2: an instance ID is reusable only once the existing workflow and every child workflow it created, checked recursively, has reached a terminal state (COMPLETED, FAILED, or TERMINATED); otherwise the create request is rejected with a 409 conflict naming the child workflow that is not yet terminal. Purging a workflow (or a retention policy) frees its instance ID. During a rolling upgrade, children hosted by older sidecars are verified only for their own status, so the recursive guarantee applies once all sidecars run a version with the check.

Also update the breaking-changes page: the 1.18.0 removal entry now notes that SDK options still exposing the reuse policy (such as reuse_id_policy in the Python SDK) are deprecated and silently ignored by 1.18+ runtimes, and a new 1.18.2 entry records the terminal-tree requirement.

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