Documentation/hw-vuln: Add documentation for Cross-Thread Return Predictions

commit 493a2c2d upstream.

Add the admin guide for the Cross-Thread Return Predictions vulnerability.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <60f9c0b4396956ce70499ae180cb548720b25c7e.1675956146.git.thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>