Removed some out of date documentation, cleaned things up

src/device_handle/async_api.rs

@@ -88,10 +88,8 @@ impl<C: UsbContext> AsyncTransfer<C> {
         result
     }
 
-    // We need to invoke our closure using a c-style function, so we store the closure
-    // inside the custom user data field of the transfer struct, and then call the
-    // user provided closure from there.
-    // Step 4 of async API
+    // Part of step 4 of async API the transfer is finished being handled when
+    // `poll()` is called.
     extern "system" fn transfer_cb(transfer: *mut ffi::libusb_transfer) {
         // Safety: libusb should never make this null, so this is fine
         let transfer = unsafe { &mut *transfer };
@@ -112,11 +110,17 @@ impl<C: UsbContext> AsyncTransfer<C> {
             .push_back(transfer.pool_id);
     }
 
+    // Step 3 of async API
     fn submit(self: &mut Transfer<C>) -> Result<(), AsyncError> {
         todo!()
     }
 }
-// TODO: Figure out how to destroy transfers
+impl<C: UsbContext> Drop for AsyncTransfer<C> {
+    fn drop(&mut self) {
+        // TODO: Figure out how to destroy transfers, which is step 5 of async API.
+        todo!()
+    }
+}
 
 /// Represents a pool of asynchronous transfers, that can be polled to completion
 pub struct AsyncPool<C: UsbContext> {
@@ -158,7 +162,7 @@ impl<C: UsbContext> AsyncPool<C> {
     }
 
     /// Once a transfer is completed, check the c struct for errors, otherwise swap
-    /// buffers
+    /// buffers. Step 4 of async API.
     fn handle_completed_transfer(
         transfer: &mut Transfer<C>,
         new_buf: Vec<u8>,