docs(sdcardio): clarify SD-first init rule for boards with floating CS pins

The existing "Important" note says the SD card must always be initialized before any other SPI peripheral. This is correct in most cases, but is misleading for boards like the Feather RP2040 RFM where the co-located peripheral (RFM95) has a floating CS pin with no hardware pull-up. On those boards the floating CS corrupts the SPI bus during SD init, so that peripheral's CS must be driven HIGH first. Added an exception paragraph to the important block to document this case.

Author: mikeysklar

shared-bindings/sdcardio/SDCard.c

@@ -43,6 +43,12 @@
 //|            Failure to do so can prevent the SD card from being recognized until it is
 //|            powered off or re-inserted.
 //|
+//|         Exception: on boards where another SPI peripheral has a floating CS
+//|         pin with no hardware pull-up (such as the Feather RP2040 RFM), that
+//|         peripheral's CS must be driven HIGH before SD card initialization.
+//|         Failure to do so will corrupt the SPI bus during SD card init. In
+//|         these cases, initialize and drive the other peripheral's CS high
+//|         first, then initialize the SD card.
 //|         Example usage:
 //|
 //|         .. code-block:: python