config.def.h: add a comment to prevent a user config error

tabspaces must be > 0 or else it will loop forever

config.def.h

@@ -91,7 +91,7 @@ char *termname = "st-256color";
  *
  *	stty tabs
  */
-unsigned int tabspaces = 8;
+unsigned int tabspaces = 8; /* must be > 0 */
 
 /* Terminal colors (16 first used in escape sequence) */
 static const char *colorname[] = {