Correct return value and documentation of wp_unschedule_event

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A few weeks ago, when looking through the cron implementation, I noticed that the documentation for the @return value of wp_unschedule_event() function doesn't quite match what actually gets return.

For example, the documentation is @return false|void False if the event does not get unscheduled..

But, the function really only returns false if this condition is truthy:

if ( ! is_numeric( $timestamp ) || $timestamp <= 0 )

Which means that we'd only return false if 1) the timestamp is not numeric or 2) the timestamp is negative or 0.

But, this doesn't handle the case where we attempt to unschedule a non-existent event.

I am attaching a patch that updates the documentation and adds a conditional that will return false if we attempt to unschedule a non-existent event.

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Return false if attempting to unschedule non-existent event

[peterwilsoncc]

Fixed in [43050] as part of changes made for #21072.