drivers: serial: Clarify usage of TX/RX IRQ predicates.

uart_irq_tx_empty() function proved to be problematic: its semantics
was not documented properly, and many hardware uses terminology like
"TX register empty" to signify condition of TX register being ready
to accept another character (what in Zephyr is tested with
uart_irq_tx_ready()). To avoid confusion, uart_irq_tx_empty() was
renamed to uart_irq_tx_complete(), propagating to drivers/serial
device methods.

The semantics and usage model of all of uart_irq_rx_ready(),
uart_irq_tx_ready(), uart_irq_tx_complete() is now described in
detail.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>