stronger wording

include/grpc/event_engine/event_engine.h

@@ -196,7 +196,10 @@ class EventEngine : public std::enable_shared_from_this<EventEngine>,
     /// caller must ensure that the callback has access to the buffer when it
     /// executes. Ownership of the buffer is not transferred. Either an error is
     /// passed to the callback (like socket closed), or valid data is available
-    /// in the buffer, but never both at the same time.
+    /// in the buffer, but never both at the same time. Implementations that
+    /// receive valid data must not throw that data away - that is, if valid
+    /// data is received on the underlying endpoint, a callback will be made
+    /// with that data available and an ok status.
     ///
     /// There can be at most one outstanding read per Endpoint at any given
     /// time. An outstanding read is one in which the \a on_read callback has