docs/library/os: Document dupterm_notify function.

Closes issue #17799.

Signed-off-by: Jos Verlinde <Jos_Verlinde@hotmail.com>

docs/library/os.rst

@@ -133,6 +133,37 @@ Terminal redirection and duplication
 
    The function returns the previous stream-like object in the given slot.
 
+.. function:: dupterm_notify(obj_in, /)
+
+    Notify the MicroPython REPL that input is available on a stream-like object
+    previously registered via `os.dupterm()`.
+
+    This function should be called by custom stream implementations (e.g., UART,
+    Bluetooth, or other non-USB REPL streams) to inform the REPL that input is
+    ready to be read. Proper use ensures that special characters such as
+    Ctrl+C (used to trigger KeyboardInterrupt) are processed promptly by the
+    REPL, enabling expected interruption behavior for user code.
+
+    The *obj_in* parameter is ignored by `os.dupterm_notify()`, but is required to allow calling
+    dupterm_notify from an interrupt handler such as `UART.irq()`.
+
+    Example:
+
+    .. code-block:: python
+
+        from machine import UART
+        import os
+        uart = UART(0)
+        os.dupterm(uart, 0)
+        uart.irq(os.dupterm_notify, machine.UART.IRQ_RX)
+
+    .. note::
+        If the ``dupterm_notify()`` function is not called, input from the custom stream
+        may not be detected or processed until the next REPL poll, potentially delaying
+        KeyboardInterrupts or other control signals.
+        This is especially important for UART, Bluetooth and other
+        non-standard REPL connections, where automatic notification is not guaranteed.
+
 Filesystem mounting
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