Add comments to explain why openAndTruncateStream()

app/src/main/java/org/schabi/newpipe/local/subscription/services/SubscriptionsExportService.java

@@ -76,6 +76,9 @@ public class SubscriptionsExportService extends BaseImportExportService {
 
         try {
             outFile = new StoredFileHelper(this, path, "application/json");
+            // truncate the file before writing to it, otherwise if the new content is smaller than
+            // the previous file size, the file will retain part of the previous content and be
+            // corrupted
             outputStream = new SharpOutputStream(outFile.openAndTruncateStream());
         } catch (final IOException e) {
             handleError(e);