Add Thermistor 20-21 notes (#19246)

Marlin/Configuration.h

@@ -375,8 +375,10 @@
  *    13 : 100k Hisens 3950  1% up to 300°C for hotend "Simple ONE " & "Hotend "All In ONE"
  *    15 : 100k thermistor calibration for JGAurora A5 hotend
  *    18 : ATC Semitec 204GT-2 (4.7k pullup) Dagoma.Fr - MKS_Base_DKU001327
- *    20 : Pt100 with circuit in the Ultimainboard V2.x with 5v excitation (AVR)
+ *    20 : Pt100 with circuit in the Ultimainboard V2.x with mainboard ADC reference voltage = INA826 amplifier-board supply voltage.
- *    21 : Pt100 with circuit in the Ultimainboard V2.x with 3.3v excitation (STM32 \ LPC176x....)
+ *         NOTES: (1) Must use an ADC input with no pullup. (2) Some INA826 amplifiers are unreliable at 3.3V so consider using sensor 147, 110, or 21.
+ *    21 : Pt100 with circuit in the Ultimainboard V2.x with 3.3v ADC reference voltage (STM32, LPC176x....) and 5V INA826 amplifier board supply.
+ *         NOTE: ADC pins are not 5V tolerant. Not recommended because it's possible to damage the CPU by going over 500°C.
  *    22 : 100k (hotend) with 4.7k pullup to 3.3V and 220R to analog input (as in GTM32 Pro vB)
  *    23 : 100k (bed) with 4.7k pullup to 3.3v and 220R to analog input (as in GTM32 Pro vB)
  *    30 : Kis3d Silicone heating mat 200W/300W with 6mm precision cast plate (EN AW 5083) NTC100K / B3950 (4.7k pullup)

Marlin/src/module/thermistor/thermistor_21.h

@@ -26,7 +26,8 @@
 #undef OV_SCALE
 #define OV_SCALE(N) (float((N) * 5) / 3.3f)
 
-// Pt100 with INA826 amp with 3.3v excitation based on "Pt100 with INA826 amp on Ultimaker v2.0 electronics"
+// Pt100 with INA826 amplifier board with 5v supply based on Thermistor 20, with 3v3 ADC reference on the mainboard.
+// If the ADC reference and INA826 board supply voltage are identical, Thermistor 20 instead.
 const temp_entry_t temptable_21[] PROGMEM = {
   { OV(  0),    0 },
   { OV(227),    1 },