Documentation - clarify that tasks will run at the specified time or later

[kkmuffme]

From the docs it seems like that task are guaranteed to run at the scheduled time - however practically (when you have larger queues) this is never the case.
(besides the fact that the worker might not be running (since it depends on cron or wp-admin requests to get started)

Even on small-ish queues with fast actions (e.g. send a 5k newsletters) and multiple workers some of the tasks will execute up to 30 minutes later than scheduled.

While this isn't an issue for many applications and also how WP cron works, this isn't clear from the docs at all.

[coreymckrill]

@kkmuffme thanks for the feedback. Where in the docs do you think would be best to put a clarification like this?