x86/fred: Enable FRED by default on AMD systems

FRED is now believed to be complete for AMD systems, and has had its tyres
kicked by both XenServer and AMD.  Enable FRED by default on capable AMD
systems (Zen6 and later).

Support on Intel is still not yet complete.  Leave it as tech preview and not
security supported.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

docs/misc/xen-command-line.pandoc

@@ -1259,12 +1259,13 @@ does not provide `VM_ENTRY_LOAD_GUEST_PAT`.
 ### fred (x86)
 > `= <bool>`
 
-> Default: `false`
+> Default: `true` on AMD, `false` otherwise
 
 Flexible Return and Event Delivery is an overhaul of interrupt, exception and
-system call handling, fixing many corner cases in the x86 architecture, and
-expected in hardware from 2025.  Support in Xen is a work in progress and
-disabled by default.
+system call handling, fixing many corner cases in the x86 architecture, and is
+available on Intel Panther Lake and Diamond Rapids CPUs, and AMD Zen6 CPUs.
+FRED is fully supported on AMD hardware.  On Intel hardware it is still tech
+preview, and in particular not security supported.
 
 ### gnttab
 > `= List of [ max-ver:<integer>, transitive=<bool>, transfer=<bool> ]`