drivers: serial: Clarification for uart_fifo_fill()/read() calls

As they are part of interrupt-driver API, they must be called from
an ISR. That means that calling it outside IST may not have a desired
effect, and vice-versa, not calling them from ISR can lead to issues.

The patch also eleborates/fixes description of uart_irq_rx_ready().

Jira: ZEP-2016

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