Explain the signature.

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@3936 d4fa192b-c00b-0410-8231-f00ffab90ce4

bin/varnishd/storage_persistent.c

@@ -133,6 +133,11 @@ static VTAILQ_HEAD(,smp_sc)	silos = VTAILQ_HEAD_INITIALIZER(silos);
 
 /*********************************************************************
  * SIGNATURE functions
+ * The signature is SHA256 over:
+ *    1. The smp_sign struct up to but not including the length field.
+ *    2. smp_sign->length bytes, starting after the smp_sign structure
+ *    3. The smp-sign->length field.
+ * The signature is stored after the byte-range from step 2.
  */
 
 #define SIGN_DATA(ctx)	((void *)((ctx)->ss + 1))