Add note on cleanup of S3 multipart uploads (#77619)

* Add note on cleanup of S3 multipart uploads

Adds docs describing Elasticsearch's behaviour around leaking multipart uploads
and what to do about it.

Closes #44955

* instance -> example

docs/plugins/repository-s3.asciidoc

@@ -453,6 +453,33 @@ bucket, in this example, named "foo".
 The bucket needs to exist to register a repository for snapshots. If you did not
 create the bucket then the repository registration will fail.
 
+===== Cleaning up multi-part uploads
+
+{es} uses S3's multi-part upload process to upload larger blobs to the
+repository. The multi-part upload process works by dividing each blob into
+smaller parts, uploading each part independently, and then completing the
+upload in a separate step. This reduces the amount of data that {es} must
+re-send if an upload fails: {es} only needs to re-send the part that failed
+rather than starting from the beginning of the whole blob. The storage for each
+part is charged independently starting from the time at which the part was
+uploaded.
+
+If a multi-part upload cannot be completed then it must be aborted in order to
+delete any parts that were successfully uploaded, preventing further storage
+charges from accumulating. {es} will automatically abort a multi-part upload on
+failure, but sometimes the abort request itself fails. For example, if the
+repository becomes inaccessible or the instance on which {es} is running is
+terminated abruptly then {es} cannot complete or abort any ongoing uploads.
+
+You must make sure that failed uploads are eventually aborted to avoid
+unnecessary storage costs. You can use the
+https://docs.aws.amazon.com/AmazonS3/latest/API/API_ListMultipartUploads.html[List
+multipart uploads API] to list the ongoing uploads and look for any which are
+unusually long-running, or you can
+https://docs.aws.amazon.com/AmazonS3/latest/userguide/mpu-abort-incomplete-mpu-lifecycle-config.html[configure
+a bucket lifecycle policy] to automatically abort incomplete uploads once they
+reach a certain age.
+
 [[repository-s3-aws-vpc]]
 [discrete]
 ==== AWS VPC Bandwidth Settings