clarify comment

crypto/rand/md_rand.c

@@ -492,12 +492,15 @@ static int ssleay_rand_bytes(unsigned char *buf, int num)
 			goto err;
 
 #ifndef PURIFY /* purify complains */
-		/* DO NOT REMOVE THE FOLLOWING CALL TO MD_Update()! */
+		/* The following line uses the supplied buffer as a small
+		 * source of entropy: since this buffer is often uninitialised
+		 * it may cause programs such as purify or valgrind to
+		 * complain. So for those builds it is not used: the removal
+		 * of such a small source of entropy has negligible impact on
+		 * security.
+		 */
 		if (!MD_Update(&m,buf,j))
 			goto err;
-		/* We know that line may cause programs such as
-		   purify and valgrind to complain about use of
-		   uninitialized data.  */
 #endif
 
 		k=(st_idx+MD_DIGEST_LENGTH/2)-st_num;