document return values

http-body/src/lib.rs

@@ -53,12 +53,18 @@ pub trait Body {
     /// Attempt to progress the body's state without pulling a new frame.
     ///
     /// `Body` consumers can use this method to allow the `Body` implementation to continue to
-    /// perform work even when the consumer is not yet ready to read the next frame.  For example,
+    /// perform work even when the consumer is not yet ready to read the next frame. For example,
     /// a `Body` implementation could maintain a timer counting down between `poll_frame` calls and
     /// report an error from `poll_progress` when time expires.
     ///
     /// Consumers are *not* required to call this method. A `Body` implementation should not depend
     /// on calls to `poll_progress` to occur.
+    ///
+    /// An error returned from this method is considered to be equivalent to an error returned from
+    /// `poll_frame`.
+    ///
+    /// Implementations must allow additional calls to this method after it returns
+    /// `Poll::Ready(Ok(())`.
     fn poll_progress(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
         let _ = cx;
         Poll::Ready(Ok(()))