bcachefs: comment bch_subvolume

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
diff --git a/fs/bcachefs/bcachefs_format.h b/fs/bcachefs/bcachefs_format.h
index a760361..6abd19c 100644
--- a/fs/bcachefs/bcachefs_format.h
+++ b/fs/bcachefs/bcachefs_format.h
@@ -1123,6 +1123,9 @@
 	 * Snapshot subvolumes form a tree, separate from the snapshot nodes
 	 * tree - if this subvolume is a snapshot, this is the ID of the
 	 * subvolume it was created from:
+	 *
+	 * This is _not_ necessarily the subvolume of the directory containing
+	 * this subvolume:
 	 */
 	__le32			parent;
 	__le32			pad;