[PATCH] x86: document sysenter path

This path isn't obvious.  It looks as if the kernel will be taking three
args from the user stack, but it only takes one from there.

Signed-off-by: Albert Cahalan <acahalan@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>