target/arm/translate.h: expand comment on DISAS_EXIT

We already have an exit condition, DISAS_UPDATE which will exit the
run-loop. Expand on the difference with DISAS_EXIT in the comments.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-id: 20170713141928.25419-4-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/translate.h

@@ -140,7 +140,10 @@ static void disas_set_insn_syndrome(DisasContext *s, uint32_t syn)
  */
 #define DISAS_BX_EXCRET 11
 /* For instructions which want an immediate exit to the main loop,
- * as opposed to attempting to use lookup_and_goto_ptr.
+ * as opposed to attempting to use lookup_and_goto_ptr. Unlike
+ * DISAS_UPDATE this doesn't write the PC on exiting the translation
+ * loop so you need to ensure something (gen_a64_set_pc_im or runtime
+ * helper) has done so before we reach return from cpu_tb_exec.
  */
 #define DISAS_EXIT 12