Add documentation about the ioctl documentation feature

Author: roblabla

src/sys/ioctl/mod.rs

@@ -97,6 +97,16 @@
 //! Most `ioctl`s have no or little documentation. You'll need to scrounge through
 //! the source to figure out what they do and how they should be used.
 //!
+//! How do I document the generated functions?
+//! ==========================================
+//!
+//! ```
+//! ioctl! {
+//!     /// This is a binding to the blkgetsize64 ioctl
+//!     read blkgetsize64 with 0x12, 114; u64
+//! }
+//! ```
+//!
 #[cfg(any(target_os = "linux", target_os = "android"))]
 #[path = "platform/linux.rs"]
 #[macro_use]