API docs: describe libusb_transfer_cb_fn type

Add some text describing the libusb_transfer_cb_fn function type
with the semantics I have gathered from reading other parts of
the API docs, referring to the proper section for more details.

[dsd: tweaked the description slightly]

diff --git a/libusb/libusb.h b/libusb/libusb.h
index fab053e..7699138 100644 (file)
--- a/libusb/libusb.h
+++ b/libusb/libusb.h
@@ -690,6 +690,15 @@ struct libusb_iso_packet_descriptor {
 
 struct libusb_transfer;
 
+/** \ingroup asyncio
+ * Asynchronous transfer callback function type. When submitting asynchronous
+ * transfers, you pass a pointer to a callback function of this type via the
+ * \ref libusb_transfer::callback "callback" member of the libusb_transfer
+ * structure. libusb will call this function later, when the transfer has
+ * completed or failed. See \ref asyncio for more information.
+ * \param transfer The libusb_transfer struct the callback function is being
+ * notified about.
+ */
 typedef void (*libusb_transfer_cb_fn)(struct libusb_transfer *transfer);
 
 /** \ingroup asyncio