Make dma module public (#583)

I was trying to use DMA in a less trivial case than the examples and
couldn't because I couldn't write the types (since I'd forgotten to
export the module).

This will likely just be 0.19.2 with the extra `pub mod` :)

- [x] Changelog updated

agb/src/dma.rs

@@ -57,6 +57,26 @@ impl Dma {
         unsafe { MemoryMapped::new(dma_control_addr(self.number)) }.set(0);
     }
 
+    /// Triggers a transfer from `values` to `location` to happen on every horizontal blank.
+    ///
+    /// This is useful for doing things like circular windows, wobbly backgrounds or fades
+    /// in palettes that you'd want to change every scan line.
+    ///
+    /// `values` must be a slice of length at lest 160 (since that is how many visible lines there are)
+    /// and the first value will be copied to the target address immediately since the items are only
+    /// transferred at the _end_ of the hblank line rather than the beginning.
+    ///
+    /// It is recommended that you set this up as soon after the vblank interrupt as possible, and don't
+    /// drop the DmaTransferHandler return value until the next vblank interrupt to ensure that you
+    /// a continuous effect.
+    ///
+    /// # Safety
+    ///
+    /// While DmaTransferHandle is not dropped, the slice at `values` must not move in memory.
+    ///
+    /// # Examples
+    ///
+    /// See the `dma_effect_*` examples in the repository to see some ways to use this.
     pub unsafe fn hblank_transfer<'a, T>(
         &'a self,
         location: &DmaControllable<T>,

agb/src/lib.rs

@@ -153,7 +153,8 @@ mod agbabi;
 mod bitarray;
 /// Implements everything relating to things that are displayed on screen.
 pub mod display;
-mod dma;
+/// Provides access to the GBA's direct memory access (DMA) which is used for advanced effects
+pub mod dma;
 /// Button inputs to the system.
 pub mod input;
 /// Interacting with the GBA interrupts