doc: uart: Make sure uart_poll_in is non-blocking

The documentation has wrongly stated that the function uart_poll_in() is
also a blocking function.  The uart_poll_out() is indeed a blocking
function but uart_poll_in() has never been since day one.

Make it clear that the uart_poll_in() is a NON-blocking function, and
uart_poll_out() IS a blocking.

This fixes #45468.

Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>