Improve bignum documentation

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/library/constant_time.c b/library/constant_time.c
index cd3ec10..d2649ce 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -749,9 +749,10 @@
                                  size_t len )
 {
     size_t i;
-    /* The value of any of these variables is either 0 or 1 at all times. */
     unsigned ret, cond, done;
 
+    /* The value of any of these variables is either 0 or 1 for the rest of
+     * their scope. */
     ret = cond = done = 0;
 
     for( i = len; i > 0; i-- )