Add more comment on the modified line

Author: bcostm

targets/TARGET_STM/TARGET_STM32F4/analogin_api.c

@@ -183,7 +183,12 @@ static inline uint16_t adc_read(analogin_t *obj)
             return 0;
     }
 
-    ADC->CCR &= ~(ADC_CCR_VBATE | ADC_CCR_TSVREFE); // Workaround
+    // Measuring VBAT sets the ADC_CCR_VBATE bit in ADC->CCR, and there is not
+    // possibility with the ST HAL driver to clear it. If it isn't cleared,
+    // VBAT remains connected to the ADC channel in preference to temperature,
+    // so VBAT readings are returned in place of temperature.
+    ADC->CCR &= ~(ADC_CCR_VBATE | ADC_CCR_TSVREFE);
+
     HAL_ADC_ConfigChannel(&AdcHandle, &sConfig);
 
     HAL_ADC_Start(&AdcHandle); // Start conversion