Mention that timeout accessors aren't always available

Author: sfackler

src/libstd/net/tcp.rs

@@ -164,6 +164,10 @@ impl TcpStream {
     /// Returns the read timeout of this socket.
     ///
     /// If the timeout is `None`, then `read` calls will block indefinitely.
+    ///
+    /// # Note
+    ///
+    /// Some platforms do not provide access to the current timeout.
     #[unstable(feature = "socket_timeout", reason = "RFC 1047 - recently added")]
     pub fn read_timeout(&self) -> io::Result<Option<Duration>> {
         self.0.read_timeout()
@@ -172,6 +176,10 @@ impl TcpStream {
     /// Returns the write timeout of this socket.
     ///
     /// If the timeout is `None`, then `write` calls will block indefinitely.
+    ///
+    /// # Note
+    ///
+    /// Some platforms do not provide access to the current timeout.
     #[unstable(feature = "socket_timeout", reason = "RFC 1047 - recently added")]
     pub fn write_timeout(&self) -> io::Result<Option<Duration>> {
         self.0.write_timeout()