service/s3/s3manager: Add doc for sequential download (#2065)

Adds documentation for downloading object sequentially with the S3 download manager.

go.mod

@@ -0,0 +1,6 @@
+module github.com/aws/aws-sdk-go
+
+require (
+ github.com/go-ini/ini v1.25.4
+ github.com/jmespath/go-jmespath v0.0.0-20160202185014-0b12d6b521d8
+)

go.sum

@@ -0,0 +1,2 @@
+github.com/go-ini/ini v1.25.4/go.mod h1:ByCAeIL28uOIIG0E3PJtZPDL8WnHpFKFOtgjp+3Ies8=
+github.com/jmespath/go-jmespath v0.0.0-20160202185014-0b12d6b521d8/go.mod h1:Nht3zPeWKUH0NzdCt2Blrr5ys8VGpn0CEB0cQHVjt7k=

service/s3/s3manager/download.go

@@ -39,6 +39,8 @@ type Downloader struct {
  // The number of goroutines to spin up in parallel when sending parts.
  // If this is set to zero, the DefaultDownloadConcurrency value will be used.
  //
+ // Concurrency of 1 will download the parts sequentially.
+ //
  // Concurrency is ignored if the Range input parameter is provided.
  Concurrency int
 
@@ -135,6 +137,9 @@ type maxRetrier interface {
 // The w io.WriterAt can be satisfied by an os.File to do multipart concurrent
 // downloads, or in memory []byte wrapper using aws.WriteAtBuffer.
 //
+// Specifying a Downloader.Concurrency of 1 will cause the Downloader to
+// download the parts from S3 sequentially.
+//
 // If the GetObjectInput's Range value is provided that will cause the downloader
 // to perform a single GetObjectInput request for that object's range. This will
 // caused the part size, and concurrency configurations to be ignored.
@@ -160,6 +165,9 @@ func (d Downloader) Download(w io.WriterAt, input *s3.GetObjectInput, options ..
 // The w io.WriterAt can be satisfied by an os.File to do multipart concurrent
 // downloads, or in memory []byte wrapper using aws.WriteAtBuffer.
 //
+// Specifying a Downloader.Concurrency of 1 will cause the Downloader to
+// download the parts from S3 sequentially.
+//
 // It is safe to call this method concurrently across goroutines.
 //
 // If the GetObjectInput's Range value is provided that will cause the downloader