add caveat about software RS485 implementations

Author: dhalbert

shared-bindings/busio/UART.c

@@ -63,7 +63,12 @@
 //|         :param int receiver_buffer_size: the character length of the read buffer (0 to disable). (When a character is 9 bits the buffer will be 2 * receiver_buffer_size bytes.)
 //|
 //|         *New in CircuitPython 4.0:* ``timeout`` has incompatibly changed units from milliseconds to seconds.
-//|         The new upper limit on ``timeout`` is meant to catch mistaken use of milliseconds."""
+//|         The new upper limit on ``timeout`` is meant to catch mistaken use of milliseconds.
+//|
+//|         .. note:: RS485 support on i.MX and Raspberry Pi RP2040 is implemented in software.
+//|            The timing for the ``rs485_dir`` pin signal is done on a best-effort basis, and may not meet
+//|            RS485 specifications intermittently.
+//|         """
 //|         ...
 //|
 typedef struct {