Clarify multipart URL comments: filer address not used for uploads

Update comments to reflect that multipart upload URLs are not actually
used for upload traffic - uploads go directly to volume servers.

Key clarifications:
- genPartUploadUrl: Filer address is parsed out, only path is used
- CompleteMultipartUpload Location: Informational field per AWS S3 spec
- Actual uploads bypass filer proxy and go directly to volume servers

The filer address in these URLs is NOT a HA concern because:
1. Part uploads: URL is parsed for path, upload goes to volume servers
2. Location URL: Informational only, clients use S3 endpoint

This addresses the observation that S3 uploads don't go through filers,
only metadata operations do.

weed/s3api/filer_multipart.go

@@ -183,9 +183,9 @@ func (s3a *S3ApiServer) completeMultipartUpload(r *http.Request, input *s3.Compl
 		entryName, dirName := s3a.getEntryNameAndDir(input)
 		if entry, _ := s3a.getEntry(dirName, entryName); entry != nil && entry.Extended != nil {
 			if uploadId, ok := entry.Extended[s3_constants.SeaweedFSUploadId]; ok && *input.UploadId == string(uploadId) {
-				// Note: Location URL points to current active filer tracked by FilerClient
-				// Clients should use S3 API endpoint, not this direct URL
-				// If direct access is attempted and filer is down, request will fail
+				// Note: Location URL is informational per AWS S3 API spec
+				// Clients should use the S3 API endpoint, not this direct URL
+				// Points to current active filer (not a concern since it's not used for actual access)
 				return &CompleteMultipartUploadResult{
 					Location: aws.String(fmt.Sprintf("http://%s%s/%s", s3a.getFilerAddress().ToHttpAddress(), urlEscapeObject(dirName), urlPathEscape(entryName))),
 					Bucket:   input.Bucket,

weed/s3api/s3api_object_handlers_multipart.go

@@ -438,10 +438,9 @@ func (s3a *S3ApiServer) genUploadsFolder(bucket string) string {
 }
 
 func (s3a *S3ApiServer) genPartUploadUrl(bucket, uploadID string, partID int) string {
-	// Note: URL points to current active filer tracked by FilerClient
-	// If this filer becomes unavailable, clients uploading parts will fail
-	// TODO: Consider using a virtual hostname that load-balances to healthy filers,
-	// or have S3 server proxy the upload to a healthy filer
+	// Note: This URL is used internally to generate the file path for putToFiler
+	// The filer address is parsed out - actual upload goes directly to volume servers
+	// The filer address here is not actually used for upload traffic
 	return fmt.Sprintf("http://%s%s/%s/%04d_%s.part",
 		s3a.getFilerAddress().ToHttpAddress(), s3a.genUploadsFolder(bucket), uploadID, partID, uuid.NewString())
 }