Clarify behavior when dropping an active output-stream

wit/streams.wit

@@ -102,6 +102,10 @@ interface streams {
     /// promptly, which could even be zero. To wait for the stream to be ready to
     /// accept data, the `subscribe` function to obtain a `pollable` which can be
     /// polled for using `wasi:io/poll`.
+    ///
+    /// Dropping an `output-stream` while there's still an active write in
+    /// progress may result in the data being lost. Before dropping the stream,
+    /// be sure to fully flush your writes.
     resource output-stream {
         /// Check readiness for writing. This function never blocks.
         ///