document ainc8 botch (ticket locks use it incorrectly)

Reference: /n/atom/patch/applied2013/ainc8botch
Date: Sun Dec 29 15:53:36 CET 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/k10/l64v.s	Sun Dec 29 15:53:18 2013
+++ /sys/src/nix/k10/l64v.s	Sun Dec 29 15:53:19 2013
@@ -267,6 +267,13 @@
 /*
  * Synchronisation
  */
+TEXT ainc8(SB), 1, $-4
+	XORL	AX, AX
+	INCL	AX
+	LOCK;	XADDB AX, (RARG)
+/* BOTCH	INCL	AX */
+	RET
+
 TEXT ainc(SB), 1, $-4				/* int ainc(int*); */
 	MOVL	$1, AX
 	LOCK; XADDL AX, (RARG)
@@ -335,12 +342,6 @@
 	RET
 _cas64r0:
 	DECL	AX
-	RET
-
-TEXT ainc8(SB), 1, $-4
-	XORL	AX, AX
-	INCL	AX
-	LOCK;	XADDB AX, (BP)
 	RET
 
 /*