uerrno: Document alongside OSError.

These are meant to be used together, so document them in the same place along with a practical example.

examples/micropython/oserror.py

@@ -0,0 +1,11 @@
+from pybricks.pupdevices import Motor
+from pybricks.parameters import Port
+
+from uerrno import ENODEV
+
+try:
+    my_motor = Motor(Port.A)
+    print("Detected a motor.")
+except OSError as ex:
+    if ex.errno == ENODEV:
+        print("There is no motor this port.")