Documentation/x86: Document new attack vector controls

Document the 5 new attack vector command line options, how they
interact with existing vulnerability controls, and recommendations on when
they can be disabled.

Note that while mitigating against untrusted userspace requires both
user-to-kernel and user-to-user protection, these are kept separate.  The
kernel can control what code executes inside of it and that may affect the
risk associated with vulnerabilities especially if new kernel mitigations
are implemented.  The same isn't typically true of userspace.

In other words, the risk associated with user-to-user or guest-to-guest
attacks is unlikely to change over time.  While the risk associated with
user-to-kernel or guest-to-host attacks may change.  Therefore, these
controls are separated.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250709155731.3279419-1-david.kaplan@amd.com