Add warning to bidi-display-reordering doc string

This explanation was given by Eli Zaretskii on emacs-devel.
For discussion, see:
https://lists.gnu.org/archive/html/emacs-devel/2019-07/msg00294.html

* src/buffer.c (syms_of_buffer): Add warning to doc string of
bidi-display-reordering to explain that it should only be used for
debugging.

1 files changed, 3 insertions, 2 deletions

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.