Add clarification for interface_spi

Boards.h

@@ -706,6 +706,7 @@
 
   #endif
   #ifndef INTERFACE_SPI
+    // INTERFACE_SPI is only required on NRF52 platforms, as the SPI pins are set in the class constructor and not by a setter method.
     // Even if custom SPI interfaces are not needed, the array must exist to prevent compilation errors.
     #define INTERFACE_SPI
     const SPIClass interface_spi[1];