drivers: uart: clarification of poll_out blocking duration

The API docs state that the uart_poll_out is a blocking call,
but it does not specify how long a call to this function should
block the calling thread. This is described in the UART driver
documentation.

This patch clarifies the API docs and alignes it with the driver
documentation.

Signed-off-by: Hubert Miś <hubert.mis@nordicsemi.no>