Document multipart completion Location URL limitation

Addresses review comment on filer_multipart.go:187

Add comment documenting that the Location URL in CompleteMultipartUpload
response points to the current active filer (tracked by FilerClient).

Note that clients should ideally use the S3 API endpoint rather than
this direct URL. If direct access is attempted and the specific filer
is unavailable, the request will fail.

Current behavior uses the currently active/healthy filer rather than
being pinned to the first filer, providing better availability.

weed/s3api/filer_multipart.go

@@ -183,6 +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
 				return &CompleteMultipartUploadResult{
 					Location: aws.String(fmt.Sprintf("http://%s%s/%s", s3a.getFilerAddress().ToHttpAddress(), urlEscapeObject(dirName), urlPathEscape(entryName))),
 					Bucket:   input.Bucket,