kprobes: Update Documentation/kprobes.txt

The patch 125e5645("Move Kconfig.instrumentation to arch/Kconfig and
init/Kconfig") had removed the "Instrumentation Support" menu,
and the configurations under this had be moved to "General setup".
Update Documentation/kprobes.txt to reflect this change.

Signed-off-by: Li Bin <huawei.libin@huawei.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>