net: add note about into_split's drop (#2567)

This took me a bit to catch on to because I didn't really think there was any reason to investigate the individual documentation of each half. As someone dealing with TCP streams directly for first time (without previous experience from other languages) this caught me by surprise

tokio/src/net/tcp/stream.rs

@@ -652,6 +652,8 @@ impl TcpStream {
     /// Unlike [`split`], the owned halves can be moved to separate tasks, however
     /// this comes at the cost of a heap allocation.
     ///
+    /// **Note:** Dropping the write half will close the TCP stream in both directions.
+    ///
     /// [`split`]: TcpStream::split()
     pub fn into_split(self) -> (OwnedReadHalf, OwnedWriteHalf) {
         split_owned(self)