exec.c: Clarify comment about tlb_flush() flush_global parameter

Clarify the comment about tlb_flush()'s flush_global parameter,
so it is clearer what it does and why it is OK that the implementation
currently ignores it.

Reviewed-by: Andreas F=C3=A4rber <afaerber@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>