docs: improve

Co-authored-by: Andrew Lamb <[email protected]>

arrow-flight/src/decode.rs

@@ -134,7 +134,8 @@ impl FlightRecordBatchStream {
 
     /// Trailers attached to this stream.
     ///
-    /// This is only filled when the entire stream was consumed.
+    /// Note that this will return `None` until the entire stream is consumed.
+    /// Only after calling `next()` returns `None`, might any available trailers be returned.
     pub fn trailers(&self) -> Option<MetadataMap> {
         self.trailers.as_ref().and_then(|trailers| trailers.get())
     }