Documentation/hw-vuln: Document the interaction between IBRS and STIBP

Documentation/hw-vuln: Document the interaction between IBRS and STIBP

commit 5ea3576ce4bff7c4ffe8d47efd0e3e44513ca690 upstream.

Explain why STIBP is needed with legacy IBRS as currently implemented
(KERNEL_IBRS) and why STIBP is not needed when enhanced IBRS is enabled.

Fixes: c3c18d0c9add ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
Signed-off-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230227060541.1939092-2-kpsingh@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>