add transfer manager doc header (#2990)

feature/s3/transfermanager/doc.go

@@ -0,0 +1,31 @@
+// Package transfermanager implements the Amazon S3 Transfer Manager, a
+// high-level S3 client library.
+//
+// Package transfermanager is the new iteration of the original
+// [feature/s3/manager] module implemented for the AWS SDK Go v2.
+//
+// # Beta
+//
+// This module is currently in a BETA release state. It is not subject to the
+// same backwards-compatibility guarantees provided by the generally-available
+// (GA) AWS SDK for Go v2. Features may be added or removed without warning,
+// and APIs may break.
+//
+// For the current GA transfer manager for AWS SDK Go v2, see
+// [feature/s3/manager].
+//
+// # Features
+//
+// Package transfermanager implements a high-level S3 client with support for the
+// following:
+//   - [Client.PutObject] - enhanced object write support w/ automatic
+//     multipart upload for large objects
+//
+// The package also exposes several opt-in hooks that configure an
+// http.Transport that may convey performance/reliability enhancements in
+// certain user environments:
+//   - round-robin DNS ([WithRoundRobinDNS])
+//   - multi-NIC dialer ([WithRotoDialer])
+//
+// [feature/s3/manager]: https://pkg.go.dev/github.com/aws/aws-sdk-go-v2/feature/s3/manager
+package transfermanager