Pass false returned for timed out transients through transient_$transient filter.

This brings the same behaviour as external object caches to the default database
backend.

Props johnbillion. Fixes #24685.

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Improve inline docs for function params that can be serialized when non-scalar. Fixes #18488.


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Update phpdoc for get_permalink(). props tivnet. fixes #25115.
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Heartbeat: better queue functionality: improve enqueue() and isQueued(), introduce dequeue() and getQueuedItem(). Props evansolomon, fixes #25047.
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