pp_hot.c:pp_subst: Move comment

Perl 5 has always had the ‘don't match same null twice’ comment
in pp_subst.  Originally it was a parenthetical note right below
the s == m.

71be2cb removed the parentheses.  Commit f722798 moved it further
away from s == m.  Now what it refers to is far from clear.

Author: Father Chrysostomos

pp_hot.c

@@ -2232,9 +2232,9 @@ PP(pp_subst)
 		    d += clen;
 		}
 		s = RX_OFFS(rx)[0].end + orig;
-	    } while (CALLREGEXEC(rx, s, strend, orig, s == m,
+	    } while (CALLREGEXEC(rx, s, strend, orig,
+				 s == m, /* don't match same null twice */
 				 TARG, NULL,
-				 /* don't match same null twice */
                      REXEC_NOT_FIRST|REXEC_IGNOREPOS|REXEC_FAIL_ON_UNDERFLOW));
 	    if (s != d) {
                 I32 i = strend - s;