ur: Review & expand object lifetime descripitons

[kbenzie]

Currently we have the Device handle lifetime and the Queue object lifetime sections in the Programming Guide but we do not have an overarching description of how object lifetimes and reference counting are expected to operate.

We also need to ensure the Device handle lifetime aligns with the implementations. As its currently defined the lifetime of a device hadle differs from the OpenCL specification which gives devices static lifetime but more closely aligns with the OpenCL sub-device lifetime description.