docs(object-store): add warning to flush (apache#5369)

* add warning to flush

* Update object_store/src/lib.rs

Co-authored-by: Raphael Taylor-Davies <1781103+tustvold@users.noreply.github.com>

* format

---------

Co-authored-by: Raphael Taylor-Davies <1781103+tustvold@users.noreply.github.com>

object_store/src/lib.rs

@@ -551,6 +551,15 @@ pub trait ObjectStore: std::fmt::Display + Send + Sync + Debug + 'static {
     /// writer fails or panics, you must call [ObjectStore::abort_multipart]
     /// to clean up partially written data.
     ///
+    /// <div class="warning">
+    /// It is recommended applications wait for any in-flight requests to complete by calling `flush`, if
+    /// there may be a significant gap in time (> ~30s) before the next write.
+    /// These gaps can include times where the function returns control to the
+    /// caller while keeping the writer open. If `flush` is not called, futures
+    /// for in-flight requests may be left unpolled long enough for the requests
+    /// to time out, causing the write to fail.
+    /// </div>
+    ///
     /// For applications requiring fine-grained control of multipart uploads
     /// see [`MultiPartStore`], although note that this interface cannot be
     /// supported by all [`ObjectStore`] backends.