diff --git a/src/buffer.c b/src/buffer.c
index 12620f0d4aa..ffd74d477bd 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5626,8 +5626,9 @@ This variable is never applied to a way of decoding a file while reading it.  */
 
   DEFVAR_PER_BUFFER ("bidi-display-reordering",
 		     &BVAR (current_buffer, bidi_display_reordering), Qnil,
-		     doc: /* Non-nil means reorder bidirectional text for display in the visual order.  */);
-
+		     doc: /* Non-nil means reorder bidirectional text for display in the visual order.
+Setting this to nil is intended for use in debugging the display code.
+Don't set to nil in normal sessions, as that is not supported.  */);
   DEFVAR_PER_BUFFER ("bidi-paragraph-start-re",
 		     &BVAR (current_buffer, bidi_paragraph_start_re), Qnil,
 		     doc: /* If non-nil, a regexp matching a line that starts OR separates paragraphs.