feat: update docs for invalid_input_files_in_subfolder (#1983)

* Updated docs for invalid_input_files_in_subfolder notice

Author: sylvansson

core/src/main/java/org/mobilitydata/gtfsvalidator/input/GtfsInput.java

@@ -64,8 +64,7 @@ public static GtfsInput createFromPath(Path path, NoticeContainer noticeContaine
             : new ZipFile(new SeekableInMemoryByteChannel(Files.readAllBytes(path)));
 
     if (hasSubfolderWithGtfsFile(path)) {
-      noticeContainer.addValidationNotice(
-          new InvalidInputFilesInSubfolderNotice(invalidInputMessage));
+      noticeContainer.addValidationNotice(new InvalidInputFilesInSubfolderNotice());
     }
     return new GtfsZipFileInput(zipFile, fileName);
   }
@@ -150,8 +149,7 @@ public static GtfsInput createFromUrlInMemory(
       String fileName = zipFile.getName().replace(".zip", "");
       if (containsGtfsFileInSubfolder(
           new ZipInputStream(new ByteArrayInputStream(outputStream.toByteArray())))) {
-        noticeContainer.addValidationNotice(
-            new InvalidInputFilesInSubfolderNotice(invalidInputMessage));
+        noticeContainer.addValidationNotice(new InvalidInputFilesInSubfolderNotice());
       }
       return new GtfsZipFileInput(
           new ZipFile(new SeekableInMemoryByteChannel(outputStream.toByteArray())), fileName);

core/src/main/java/org/mobilitydata/gtfsvalidator/notice/InvalidInputFilesInSubfolderNotice.java

@@ -7,14 +7,17 @@
 /**
  * At least 1 GTFS file is in a subfolder.
  *
- * <p>All GTFS files must reside at the root level directly.
+ * <p>All GTFS files must reside at the root level directly. There are two common cases that trigger
+ * this error:
+ *
+ * <pre>
+ * 1. The root folder (e.g., a folder called "GTFS 2020") was zipped instead of the individual
+ *    files within the folder (e.g., `calendar.txt`, `agency.txt`, etc.). This can be fixed by
+ *    zipping the files directly instead.
+ * 2. A file <a href="https://gtfs.org/documentation/schedule/reference/#dataset-files">associated
+ *    with GTFS</a> is in a subfolder rather than the root folder of the dataset, and needs to be
+ *    removed if not needed or moved to the root level.
+ * </pre>
  */
 @GtfsValidationNotice(severity = ERROR)
-public class InvalidInputFilesInSubfolderNotice extends ValidationNotice {
-  /** The error message that explains the reason for the exception. */
-  private final String message;
-
-  public InvalidInputFilesInSubfolderNotice(String message) {
-    this.message = message;
-  }
-}
+public class InvalidInputFilesInSubfolderNotice extends ValidationNotice {}