src/dispextern.h: Fix commentary for struct it's string_nchars member.

diff --git a/src/dispextern.h b/src/dispextern.h
index 365e8bc..978a23e 100644 (file)
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -2138,7 +2138,8 @@ struct it
   const unsigned char *s;
 
   /* Number of characters in the string (s, or it->string) we iterate
-     over.  */
+     over.  Used only in display_string and its subroutines; never
+     used for overlay strings and strings from display properties.  */
   ptrdiff_t string_nchars;
 
   /* Start and end of a visible region; -1 if the region is not