net: clarify the cancellation safety of the TcpStream::peek (#7305)

tokio/src/net/tcp/stream.rs

@@ -1070,6 +1070,13 @@ impl TcpStream {
     /// Successive calls return the same data. This is accomplished by passing
     /// `MSG_PEEK` as a flag to the underlying `recv` system call.
     ///
+    /// # Cancel safety
+    ///
+    /// This method is cancel safe. If the method is used as the event in a
+    /// [`tokio::select!`](crate::select) statement and some other branch
+    /// completes first, then it is guaranteed that no peek was performed, and
+    /// that `buf` has not been modified.
+    ///
     /// # Examples
     ///
     /// ```no_run