Add usage note of buffer-invisibility-spec.

* src/buffer.c (syms_of_buffer) <buffer-invisibility-spec>: Add usage
note from the lispref.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/buffer.c b/src/buffer.c
index 1570f3831ac..ef04ca7018d 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -6190,7 +6190,9 @@ If the value is a list, a text character is invisible if its `invisible'
 property is an element in that list (or is a list with members in common).
 If an element is a cons cell of the form (PROP . ELLIPSIS),
 then characters with property value PROP are invisible,
-and they have an ellipsis as well if ELLIPSIS is non-nil.  */);
+and they have an ellipsis as well if ELLIPSIS is non-nil.
+Setting this variable is very fast, much faster than scanning all the
+text in the buffer looking for properties to change.  */);
 
   DEFVAR_PER_BUFFER ("buffer-display-count",
                      &BVAR (current_buffer, display_count), Qintegerp,