stm32: add some docs.

embassy-stm32/src/dma/mod.rs

@@ -39,6 +39,13 @@ enum Dir {
     PeripheralToMemory,
 }
 
+/// "No DMA" placeholder.
+///
+/// You may pass this in place of a real DMA channel when creating a driver
+/// to indicate it should not use DMA.
+///
+/// This often causes async functionality to not be available on the instance,
+/// leaving only blocking functionality.
 pub struct NoDma;
 
 impl_peripheral!(NoDma);