How to handle conflicts between reference manual and reference design about ADC of STM32?
You must not connect a negative voltage to VREF-. The "-" only denotes that it's the lower voltage. If you check the manual, p. 209 it will state:

VREF-: The lower/negative reference voltage for the ADC,VREF- = VSSA

and for VSSA:

VSSA: Ground for analog power supply equal to VSS