add caveat in documentation

Author: dhalbert

shared-bindings/microcontroller/Processor.c

@@ -104,7 +104,11 @@ MP_PROPERTY_GETTER(mcu_processor_reset_reason_obj,
 //|     temperature: Optional[float]
 //|     """The on-chip temperature, in Celsius, as a float. (read-only)
 //|
-//|     Is `None` if the temperature is not available."""
+//|     Is `None` if the temperature is not available.
+//|
+//|     .. note :: On small SAMD21 builds without external flash,
+//|       the reported temperature has reduced accuracy and precision, to save code space.
+//|     """
 STATIC mp_obj_t mcu_processor_get_temperature(mp_obj_t self) {
     float temperature = common_hal_mcu_processor_get_temperature();
     return isnan(temperature) ? mp_const_none : mp_obj_new_float(temperature);