Comment changes.

src/keyboard.h

@@ -66,7 +66,17 @@ struct kboard
   {
     KBOARD *next_kboard;
 
-    /* The state of a prefix arg.  */
+    /* The state of a prefix arg.
+       After pressing C-u COUNT times, prefix_factor is 4^COUNT
+       and prefix_value is nil.
+       After C-u NUM, prefix_factor is nil and prefix_value is abs(NUM).
+       (prefix_factor and prefix_value are never both non-nil.)
+       prefix_sign is always either +1 or -1; a value of -1 means that
+       the actual numeric argument is the negative of what's in prefix_value,
+       or just `-' if prefix_value is nil.
+       The boolean prefix_partial means that the user is in the process
+       of building a prefix argument, so that a minus or digit key at
+       this point is handled specially.  */
     Lisp_Object prefix_factor, prefix_value;
     int prefix_sign, prefix_partial;