"Check the book"

That's available via the 'Links' button - look for the "bible" references!

The UART interrupts pretty much tell you that the UART has finished transmitting or receiving a complete byte - if you still need a more precise definition of "finished", then you'll have to read those books!

"or will it wait until a RETI occurs"

Well, it couldn't do that, could it?
That'd mean that the processor would have to know when the RETI was coming, so that it could work out when to start transmitting in time for the byte to finish at the RETI!!!