Correct and improve comments for IFF_* constants.

Author: mwanner

src/net/if_.rs

@@ -192,10 +192,10 @@ libc_bitflags!(
         IFF_PRIVATE;
 
         // bitmask: 0x010000
-        /// FIIXME: what's this?!?
+        /// Driver signals L1 up. Volatile.
         #[cfg(any(target_os = "fuchsia", target_os = "linux"))]
         IFF_LOWER_UP;
-        /// FIIXME: what's this?!?
+        /// Interface is in polling mode.
         #[cfg(any(target_os = "dragonfly"))]
         IFF_POLLING_COMPAT;
         /// Unconfigurable using ioctl(2).
@@ -206,6 +206,7 @@ libc_bitflags!(
         IFF_NOXMIT;
 
         // bitmask: 0x020000
+        /// Driver signals dormant. Volatile.
         #[cfg(any(target_os = "fuchsia", target_os = "linux"))]
         IFF_DORMANT;
         /// User-requested promisc mode.
@@ -216,6 +217,7 @@ libc_bitflags!(
         IFF_NOLOCAL;
 
         // bitmask: 0x040000
+        /// Echo sent packets. Volatile.
         #[cfg(any(target_os = "fuchsia", target_os = "linux"))]
         IFF_ECHO;
         /// User-requested monitor mode.