Update the ADC driver docs for RP2040

docs/adc_driver.md

@@ -43,6 +43,8 @@ Then place this include at the top of your code:
 
 ### ARM
 
+#### STM32
+
 Note that some of these pins are doubled-up on ADCs with the same channel. This is because the pins can be used for either ADC.
 
 Also note that the F0 and F3 use different numbering schemes. The F0 has a single ADC and the channels are 0-indexed, whereas the F3 has 4 ADCs and the channels are 1-indexed. This is because the F0 uses the `ADCv1` implementation of the ADC, whereas the F3 uses the `ADCv3` implementation.
@@ -121,6 +123,21 @@ Also note that the F0 and F3 use different numbering schemes. The F0 has a singl
 
 <sup>² Not all STM32F4xx devices have ADC2 and/or ADC3, therefore some configurations shown in this table may be unavailable; in particular, pins `F4`…`F10` cannot be used as ADC inputs on devices which do not have ADC3. Check the device datasheet to confirm which pin functions are supported.</sup>
 
+#### RP2040
+
+RP2040 has only a single ADC (`ADCD1` in ChibiOS); in the QMK API the index for that ADC is 0.
+
+|Channel|Pin                |
+|-------|-------------------|
+|0      |`GP26`             |
+|1      |`GP27`             |
+|2      |`GP28`             |
+|3      |`GP29`             |
+|4      |Temperature sensor¹|
+
+
+<sup>¹ The temperature sensor is disabled by default and needs to be enabled by the RP2040-specific function: `adcRPEnableTS(&ADCD1)`.  The ADC must be initialized before calling that function; an easy way to ensure that is to perform a dummy conversion.</sup>
+
 ## Functions
 
 ### AVR