Documentation/hw-vuln: Update spectre doc

commit 06cb31cc761823ef444ba4e1df11347342a6e745 upstream.

commit 7c693f54c873691 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")

adds the "ibrs " option  in
Documentation/admin-guide/kernel-parameters.txt but omits it to
Documentation/admin-guide/hw-vuln/spectre.rst, add it.

Signed-off-by: Lin Yujun <linyujun809@huawei.com>
Link: https://lore.kernel.org/r/20220830123614.23007-1-linyujun809@huawei.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>